I think that tanks are really pretty necessary to simulate accurately what war was like by the end of the period. Besides use in Europe, imagine what might happen in alternate history scenarios - like Turtledove's alternate history Civil War books, it's going to be entirely possible in this game that the CSA would still exist by WWI and war with the USA around that time, and tanks in the wide-open spaces of western America would have made a HUGE change to how war was fought, as opposed to the relatively cramped forests and trenches of Western Europe where the change was minor enough that it could possibly be dismissed...
I think that given that this isn't a flat historical simulator we need to consider what might happen if ahistorical wars occurred, and a WWI-type war in North America might well have gone on in a radically different way than it did in Europe. For that matter it's disappointing to plenty of military buffs if they don't have any representation of the beginnings of the modern military that emerged during the first world war - and planes (largely for scouting, as it's been said), tanks, and submarines WERE considered important tools; that's why they sent people to their deaths using them.
I for one will enjoy playing the end of the game substantially less if I feel like war is still more like the Franco-Prussian War than like WWI - and the world of trenches, poison gas, and military machines was really substantially different than anything that had come before it. The "war to end all wars" killed off a ridiculous proportion of Europe's young men and really was something new, and a lot of that was due to the increased mechanization of war that really started in earnest with the Gatling gun.