I think it's also worth pointing out that most light infantry use mass fire, which gives them a HUGE advantage over other infantry prior to fire-by-rank. On large units, they've got 20 more men firing than a regular 120 man line infantry unit (assuming 3 lines). And more accurately too.
Actually the numbers would be a bit off due to the flag bearer/drummer/officer, but you get the idea.

So I'd say that, in the early game at least, light infantry can serve in the line as regulars, provided you've got somewhere to pull them back to when the enemy infantry comes in (Hooray for stakes).

Of course, that only applies to states that can get light infantry prior to the final tech tier where they reside normally...