mathematically 3 terradons vs. a T3 bolt thrower crew will cause 1.75 wounds, dunno about you, but i wouldnt call that reliable. and that is pretty much an ideal situation from the LM players point of view.
the real point for me is; because you're looking at such small numbers of dice, the chances of flukeing/fluffing your roll are much higher (i.e. its much easier to get 6 misses out of 6 attacks than it it 20 out of 20).
(oh, and dont forget HE bolt thrower crew still have ASF, hitting on 3s wounding on 4s, even with only 2 attacks can still sneak a wound or two in.)
dont get me wrong, i think terradons are quite useful, but they are very expensive and fragile (both in game terms and model terms... stupid flying bases!)