You don't need a case in the same way that you don't need health insurance & rental car insurance. What I mean is if you want to go naked (caseless) that's your right, but IF something happens like a drop it's an added layer of protection. The silicon & TPU cases will cushion the phone in that event, lessening the sharp impact of a drop on a hard surface which could result in a cracked screen.
The way I look at it, if your repair/replacement cost is more than you can afford, a $10-30 case is worth it. You're looking at upwards of $500 to replace the phone.
As far as extended battery cases that work with the Tbolt & leave the kickstand usable, I believe htcpedia has one in their store that will work. I think it's a TPU case (like the high gloss silicon cases Verizon sells) that stretches enough to get it on the phone, but has a high gloss plastic feel, not a rubberized feel like on a standard silicon case.
Edit: Here's a link to the extended silicon (not TPU) case for the Tbolt.
http://shop.htcpedia.com/htc-thunderbolt-extended-battery-cases.html
It's basically a standard case that leaves the headphone & charging ports, battery hump, & kickstand open. As far as protection, I think think the cause of most cracked screens is not from landing on their back ro front, but on a corner, which would be protected by this case. If you want a TPU style extended case, try the one that JMZ768 linked to in his post. Currently I cannot find it anywhere except on that thread, but the poster mentioned that Verizon can order it for you. The part number is VZW6400SILHGBXT. Hope that helps.