Positioning units for bridge defences

[caravel]

Ashi teppo units aren't necessarily useless once they run out of ammo. If you put them in hold formation and keep them together for better morale, they can be used to delay or divert enemy units, and if you have cav or fast flanking infantry available (nods, monks), engaging with your ashi fodder can tie up a superior enemy unit long enough to get in a good flank or rear attack. If the ashis rout, no big deal as samurai morale isn't affected.

I only play SP battles (as part of a campaign), but I have had to deploy Teppo Ashigaru similarly to how you describe. This is probably due to me fielding rather too many of them however. It's not so great when you turn up for battle and it's raining heavily and your army is mostly Teppo...

I find that their lack of morale is more than made up for with their morale affecting armament - especially in defensive battles.