that sounds like a pretty good idea. the only thing, and this is only me playing Devil's Advocate here, is that the enemy would be able to do it too, which on the surface sounds fair, but then remember there are five of them and only one of you.
imagine you've spent years fighting your neighbors only to see some other daimyo, who's been very quiet for some time, suddenly spring from nowhere with superior troops (he hasn't been wasting them in province skirmishes), you must remember that the AI controlled clans don't necessarily play with the best logic (ie. doing their best to prevent neighboring enemies from massing huge armies) so the recently-corrected Horde problem would be a regular occurance.
still, i wouldn't mind having the option to simultaneously build just to see how differently the game changes