I think the bat looks weirdly out of place in comparison to the other flight forms. I still maintain that a cockatoo would have been the perfect bird form for trolls. What's better than fitting with the bird theme and having a mohawk while doing it?
Well, not to sound like a jerk, but you're wrong.
Trolls and bats have a history going back a long time in Warcraft. We've been Troll Batriders in Warcraft 3 for nearly 10 years, it's about damn time we had a troll-bat connection.
"The ends justify the means!" - http://classic.battle.net/war3/orc/units/trollbatrider.shtml