Not just onagers are good for attack, I actually found that the other siege weapons work better when you attack than when you defend. Whenever I defend with siege weapons the AI always rushes my position no matter what.

When I attack the AI only rushes sometimes. Plus as mentioned before the siege weapons are excellent for driving the enemy off the hill.

The best non-siege use for these units would probably be attacking bridges, because you can use lesser units for defending bridges.

The only other thing I use siege weapons for is killing elephants, which they don't do very well, except onagers, but even they're barely adequate, and this is with temple of mars pantheon. I need to kill at least a significant percentage of one elephant unit and the onagers maybe kill about 20-25% of them.

I tried using scorpions on elephants; the elephants just charged and stepped on them. 2 scorpions + 2 archers vs. 1 war elephant = 2 dead scorpions + 2 dead archers.