Indeed in 1.12 you cannot bribe with emissaries (that is another point of attraction for me) and Ashigaru take 2 turns to build. They cost however 1/2 a koku to maintain (unlike other infantry that cost 1koku and cavalry that cost 2 koku). I haven't played MI/WE for ages now, so can't remember how long they take to build there. In any case, yes, ashigaru can be used en masse and flank en masse at the opening stages of the game, and they are devastating as such. From the middle game onwards they need to be used a little less as the other clans get more advanced troops and an ashigaru heavy army may not do the same impression.