Torx was invented (by Camcar Textron) to come up with an internal wrenching bolt design that could handle significant amount of torque. If memory servies, the Torx head can handle about 300% of the torque of the Allen/hex head of the same size.
It's all a matter of head design and the expected amount of installation tension of the bolt. The advantage of internal wrenching is the ability to recess the bolts readily and to tuck them into smaller recesses. Also, the smaller change in size between the bolt shank and the head outside diamater means that a stronger bolt can be made due to grain flow - the best bolts for tension applications have small heads and generous underhead radii, and are installed with washers with internal bevels to accommodate the underhead radius.
All in all, the Torx bolt is really a good design - it's downfall is that it requires a specialised tool that not everyone has (but if you have a Jeep, you should get them - internal and external!)
If you want to see an example of excellent bolt design, I suggest you look up the old Whitworth bolts - they're a huge pain in the arse to work with, but the Whitworth bolts feature what was probably the best thread design ever put into production! Those damn Whitworth wrenches were a pain to get, tho...
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