Seems like they are poorly integrated with the rest of the game, however.
First of all, I think that if retinues represent professional armies, then maintenance cannot be free. A professional army in this period is basically a standing force of mercenaries, and if they don't get paid, they should stage a coup.
Also, your vassals shouldn't be totally chill with you prancing about with a 100k strong standing army that could wipe them off the map in a day - if you engage in a break-neck military build-up, your vassals should freak out and depose you.
EDIT: Actually, Rome handled this part fantastically. If you had a general in permanent command of an army, the troops would become loyal to HIM...with all sorts of fun consequences. Not exactly the same, but in a similar spirit to what I'm suggesting. Standing armies pose inherent dangers to their own government in any day and age.