It can come as something of a shock to open up your first box of insanely cheap historical plastics and find either no bases or a sheet of bases of mixed sizes and shapes. This is certainly a hidden cost for the historical gamer, though still a very small one given the price difference between fantasy and historical miniatures.
The problem stems from a lack of consistency in how different rule sets expect models to be based. This isn't simply a question of size and shape, but also the number of models assigned to each base. This is usually a question of the smallest game element that can be removed at a time. For Warhammer (Ancient Battles or English Civil War) single models can be removed, and so models often need to be based singly. The DBM/Fields of Glory family uses 'elements' bases with consistent widths (40mm or 60mm depending on scale) and varying depth with 2 or more miniatures on each depending on type. For Impetus a unit is a single element and some spectacular examples of diorama style unit basing can be found with a bit of googling. Scale also plays a part, 6 or 10mm models are usually multi-based, for 15mm and larger it can vary considerably.
Of course it isn't even that simple. The requirements of moving dozens or hundreds of models around a table mean that many players prefer to multi-base their models regardless of scale. There is a long and venerable tradition of placing 4 28mm models on a square base and then marking them with dice as casualties are removed or keeping a small pool of single based models to show them.
Different rule sets tend to emphasise different aspects of basing. For Warhammer, each individual has a consistently sized base (20x20mm for infantry, 2x50mm for cavalry) with units varying in width and depth depending on the number of models. For the DBM family, base width is consistent, and depth and the number of figures varies according to type.
In practice, the rules may not always be as strict as they seem at first. Field of Glory lists the number of figures to a base as a maximum, so in principle you can line up 3 20x20mm based figures and call it an element. This works fine if you keep track of what element is what and don't lose track of which figures go together. The problem, is that in FOG and DBM the number of figures on a base is often used as determinant of type. The only difference between regular and irregular infantry may be that the former has four models per base and the latter three. This is not an insurmountable problem, but does add an additional complication if you figures aren't based according to the regulations as you have to find another way to record the different types.
With a number of new historical rule sets coming onto the market at pretty much the same time, all are vying for the attention of existing gamers. Basing is an issue, as no player wants to have to re-base their armies to play a new game and it seems to have become compulsory to advertise your game as having no rigid basing rules even if this is not strictly the case.
Clash of Empires, for instance, is keen to emphasise that its basing standards are only recommendations. The problem is that, like Warhammer, the number of attacks a unit makes against its enemy is determined by the number of models in contact. This puts models on larger bases (or at least ones with wider base space per model) at a significant disadvantage. Fortunately, the rules do have a way around the problem.
Working out how many models may fight can become a bit confusing, especially when two opposing player use different base sizes for their miniatures. If in doubt, refer back to the recommended base sizes, remembering to apply the concept that models in corner to corner contact will fight. For example, a unit of cavalry 6-wide has a frontage of 150mm and is fighting a unit of infantry. Just divide the cavalry's frontage by the opponent's individual base size (discarding any remainder) and add 2 to determine the number of models in contact. (Clash of Empires, p56)I particularly like the use of the word 'just.' This is certainly perfectly doable, but it is not trivial, particularly in the middle of a game. Nor is it indicative of a game where "the base sizes are not critical to game play."(Clash of Empires, p8). The problem is that, as much as the rule writers may want basing not to matter, when the position of a model relative to its opponent has an impact on game play it does matter like it or not.
Of all the new rule sets, Hail Caesar probably has the best claim to be able to say that basing is not really important, because its smallest element is the unit. That is to say, individual models are not removed as casualties, casualties are recorded but the unit is destroyed in one go. The number of dice rolled by a unit in combat is determined by unit type and starting size, not by models in contact.
Hail Caesar does use unit width and depth to determine unit size, a rather abstract concept defined as standard, large, small and tiny. The width is imprecise and the number of models that should fill that width is up to the players to determine, and need not strictly be consistent. If one player wants their models mounted on 25x25mm bases and the other by 20x20mm bases it doesn't matter as long as all standard sized units have roughly the same width. Having said that, if you have you Roman Legionnaires based to Field of Glory standard you will need 12 Legionnaires to fill the width that a Warhammer-based army can fill with 9, which means FOG players are going to have to have rather more models to field the same sized army as their opponent.
Of course you could fix the number of models and fudge the difference between unit widths as long as it isn't too crazy. You would have to ensure that a particularly wide unit didn't get attacked in the front by two enemy units that were meant to be the same size as it. The bigger problem is terrain. A smaller unit is simply easy to manoeuvre through narrow gaps than a bigger one. To keep things fair players would have to look at gaps between scenery or at area terrain (such as woods) and decide in advance whether they are wide enough for a standard sized unit.
Most of these problems only really exist if players are following different basing standards. If bases are consistent then it rarely matters if they are 5mm wider than the rule book recommends. But these small issues can add up and armies based in a way inconsistent with the guide lines in the rule book invariably add an unwelcome obstacle to the smooth running of a game. Basing is an issue for historical gamers that rarely exists in fantasy and sci-fi gaming. And, unfortunately, rules writers can't simply stop it being an issue by declaring that it isn't on the back of the rule book.
*Hordes of the Things is the major exception to the Sci-Fi and Fantasy basing rules, following a similar system to DBA and DBM, but it is unusual in this respect.