The TPU case may offer more protection than you are imagining...
(your imagination I hope is where this belief comes from - let us know if your TPU case has been put to the drop test and the results!)
The TPU material seems like one of the best compromises for impact protection - hard enough that it wouldn't just mush away from an impact, but soft enough that it could absorb impact shock without transferring it all directly to the phone.
On a hard impact, a slightly flexible material will deform and absorb the shock, protecting your phone from damage - like "crumple zones" on your car. Best the car deform and absorb the shock and save your life than be rigid, transfer the shock of impact to you and damage YOU.
In the case of a phone case, if I dropped my phone hard enough to damage my TPU case, I'd be glad the phone was OK and glad it only costs me $6 to replace my case.
Most of those tough-box cases make your phone huge and are just thicker and harder plastic that make the case more likely to survive a drop. Haven't EVER seen drop test results or comparisons, but there's no getting away from physics.
Bear in mind also, on a related note, this phone is very light, so on a drop there isn't as much momentum. There was a "drop test comparison" video on youtube of the Epic Touch 4G vs iPhone 4, and with no protection at all, numerous drops, the ET4G came away relatively undamaged.
Found it:
http://www.youtube.com/watch?v=-6zlXBTREcE
If anything, check out this case:
http://www.youtube.com/watch?v=eL7PIjZbLY8
Note at the end when he takes the phone out of the case, it's a very similar material, if not also TPU, with thicker corners.
IMHO though, unless you are looking to have your phone survive a drop off a moving motorcycle or similar, you are probably fine with the TPU case.