I think it would be quite useful when photographing at a school performance. Looking back through my pics - one time I was sitting back a bit more than half way back in a small auditorium, and was shooting at up to 300mm equivalent. That was to get about 4 of the performers in the shot at once. The 135 would of course be 216 equivalent. In my case it wasn't very well lit - I shot at 1/40th, f/5.6 and ISO 3200. So with the 135L, you could shoot at f/2 AND use a tripod/monopod - and drop the ISO to 400. Without a tripod/monopod - you'd want about 1/200 and therefore still be at about ISO 2000. Therefore without stabilization and without a mono/tripod - you will get less than a stop of performance better than shooting with a stabilized, regular 70-300 zoom. But - the 135 lens is very good so the pictures should be quite superior.
For general indoors use however, I think 216mm equivalent is way too much zoom. Unless you want to capture shots of just body parts - I wouldn't be able to use it in my house.