To argue the other side, and since I installed a lightweight aftermarket STB (didn't want to add weight) where it had none, I wanted to add my input. For me, I noticed a stiffer front end when taking the twisties at a decent pace over being naked and it looks damn good IMO. Also notice a "tighter" feeling when going over a bump on just one side of the car - less flex? Hope to not have to open the engine up for anymore upgrades, but if I do, the holes are there now and won't be a big deal to remove it.
I know there is marketing in here, but here is what is posted on Maximum Motorsports' site and their description of what STBs do.
"Flexing of the strut towers, both fore/aft and side to side, causes unpredictable alignment and handling changes, lack of roll coupling, vibration and metal fatigue. The MM Strut Tower Brace keeps the strut towers from moving relative to each other, by providing a load path between the strut towers and between each strut tower and the firewall. This will prevent alignment from changing under braking and cornering, making the car more stable and predictable. In addition, the increase in rigidity from the strut tower brace will also reduce firewall and dashboard vibration, metal fatigue and noise."