In Warband you can assign units to different group manually, so most people at least have a different group for companions that are valuable and should stay out of battle. To be honest, I think I liked old system better, cause in Warband cavalry is cavalry whether they still have a horse or not - I believe best system would be mix of both.
About your companions rushing despite you order to follow - it's a vanilla M&B (but Warband have kind of it as well) thing that soldiers have certain inertia when following orders and it may take them a second or five to listen to you - best seen with Khergits, as they have fast horses. When their lords order their men to follow and do their "slowly ride around" tactic, some of them can ride through half of the map before going back to their ranks.
Also, don't keep your cavalry on follow, they act like sissies in fight when they're following. Make them charge before clashing with enemy and then break on your own and order them to follow - and it's normal that some of them will get swarmed with infantry, give infantry a chance to fight, eh? Your companions are crappy, bear with it. They're not knights and if they get swarmed, they have next to no chances with breaking on their own. Later in game my Matheld was eating above 200 denars a week and only couched lance up her butt was able to stop her, so they're definitely worth the initial pain