i use peasents in BI, becouse they are so cheap(18denarii per turn with sassanids). i think that they are good enough to keep public order, but for fight i rarely use them, just in cases when fight against outnumbered enemies, and in most cases those are rebels.
also, fronline cities are secured with some better troops, combination of 4 heavy infantry units, 3 archers and some cavalry(usualy family member), is good enough to secure citi until some real army come.
when play rtw i use 3 types of armies. first some cheap infantry to secure public order(peasents in low threat c regions, and better infantry in border cities). second good and expensive defensive armies, which i use to secure few regions of rebels, or in frontlines to defend new conquered cities. and finaly, highly expirienced and best possible units to form offensive army, and palinly there is nothing that can stand in their way. i dont use units that can be recruit just in some speciall places(elephants, or camel riders), becouse of constant need for full army, especially when fightning hordes(boy they are numerous).