The only reason I never use light infantry (other than nations that only have light infantry for missiles, like C'tis) is that while you can produce them en masse in emergencies or for specific applications, maintaining them seems like a really bad idea.
Maintaining a single LI is usually almost as bad as maintaining real HI, per unit and, for the effectiveness, it's not even close (speaking mostly for the non javelin equipped types).
A unit that is rarely useful but occasionally it is a little better than the alternatives isn't useless...but it's not really terribly useful either
- Kel