An advantage to torque boxes is that they tie the subframes into the rocker sills. The rockers are a very large structural member than runs the length of the passenger compartment and these pieces are much larger then sub frame connectors so they will tend to bend less.
However, one advantage to most sub-frame connectors is that they tie in or bisect the floor which then turns the floor pan into a giant shear plate, creating a larger structural member.
If you go way back in to the past, you'll notice most competition cars did not use the longitudal subframe connectors we are all familiar with, but instead used X members tieing all four corners of the cab together along with bisecting the floor and putting it in shear. The weld in aftermarket sub frame connectors we most familiar with were developed as a reasonably easy to install structural piece that would create the greatest amount of improvement for the least amount of cost and effort. That doesn't mean they produce the best amount of rigidity, just the best amount for the cost and effort. Torque boxes can be looked at in the same way only from an OEM manufacturers perspective. They are both bang for hte buck approaches.
