How do you propose a Battanian or Sturgian Infantry player currently deal with heavy armor? Both classes have other options like the Fiann and the Druzinnik but I do not believe in balance by making certain classes, and therefore players, useless in certain situations. With a well done perk system a player should be able to do well in whichever situation he wants, with the tradeoff being the perks he choose may not give him tools to deal with other situations.
I do not think you have seen competitive Mount and Blade before. A spearman needs to be able to deter a cav from pushing the targets the spearman is protecting. If the cav knows he can just charge and more than likely "outreach" the spearman, then the spearman cannot do his job as area denial.