I agree with you; Heather is underrated.
I think Heather's starting stats can cause many people to form an instant opinion that she's crap and isn't worth the trouble without giving her a go. If people were focusing a lot on Sothe in Part 1 and he were at the same level or higher, he'd be surpassing her in most stats by now. Heather's speed is great, but her low strength and defense can ruin her. Of course, there's also the fact that Sothe is a forced unit so you might as well bump him up so he won't be completely useless in endgame.
Still, Sothe can't be everywhere at once, so if people want to steal things they're stuck with Heather :P