I think that there are some important settings that has to be taken into consideration before wondering what "class" or type of soldier will dominate in M&B multiplayer.
1. Difficulty settings. I play with 160% difficulty. Thats with full difficulty plus directional blocking. Also, playing withaut aiming reticle will help to down power the ranged troops. In my opinion it would be quite realistic too. I suppose there are many serverside settings and modding possibilities to tweak the settings affecting game balance. This also means that everyone can probably find a server which suits their style of play
2. Character settings. Premade classes with prechosen equipment? Remember that it is not granted you can have "enough" power draw, riding and horse archery to be an effective horse archer. We are all used to playing with our "lord" characters that has enough skills to place an arrow to enemys eye from quite a distance.
In multiplayer it might be that you cannot play a horse archer that can have the best bow, enough weapon skill, power draw, horse archery, riding and armour to be HALF as effective as in single player. So, practically I am saying that playing a horse archer in singleplayer might be totally different from playing a horse archer in multiplayer.
I believe there will be quite a good balance between different troop types. For example spears and pikes are exellent against charging cavarly, but also for fighting from the second rank. Lets say that I have rhodok spearmen. First rank is using shield and sidearm and standing in a line leaving small gaps for the other rank to thrust their spears between them. The first rank can concentrate on defese and the second rank can concentrate on offense.
Two handed weapons (glaives, axes) should alse be good when used from behind shield armed infantry. Depending on how high abilities and weapon skills ranged troops have, a heavily armoured infantry with 2 handed weapon might not be that easy to take down. In singleplayer its easy to place an arrow on his head, but a human player might not run straight to you, and once again, you might not have 300 skill in archery.
Mounted troops might perform more flanking manouvers and work more as a team unlike in singleplayer. Horse archers might be very usefull in attacking formations from unexpected directions, instead of "kiting" a single enemy. It might even be, that its REALLY hard to hit a single enemy from horseback! Which in my humble opinion is as it should be.
Someone mentioned that crossbows suck. Yes, they are not as good as bows, but bows require power draw. So, if you have to choose between power strike OR power draw, crossbows might suddenly sound better.