In my opinion, there is nothing wrong with an 11.50 capable truck running in the 11.90 class. In fact, there is nothing wrong with an 11.00 capable truck running in the 11.90 class.
As to why there is such a draw to 11.90, one reason is that the chassis doesn't matter. If you added a ladder bar setup, 4-link, or had a full custom rat rod, classic car with a motor swap, etc, 11.90 allows you to run. Super Street does not allow for the above chassis modifications. So what if you launch with less than peak boost and cut a 2.1 60' time and run 11.90 @ 130 MPH, just makes for an exciting finish when running a 4x4 that cut a 1.65 60' time and crosses at 11.90 @ 113 MPH.
I don't think it would matter if the index was 12.0, 11.5, or even 11.0, it would still be a fun class. Detuning a few tenths is no big deal. An in my opinion, if you want to win, you need to be able to run quicker than the index since 99% of these vehicles are not 100% reliable. That's why most of the winners have an adjustable tuning switch or small nitrous kit that can be enabled if there is spinning or a short shift that does not allow them to run the expected 11.90 tune-up. Hit the juice for a few seconds or turn up the tuning knob to make up for the mishap and hopefully you can fender-race to the finish line.....EXCITING
The more you go to races and learn about drag racing, the more you realize it's not about setting a new personal record every run and having the rules tailored so you can run all-out every pass. It's about consistency, contingency or back-up plans for the botched runs, short shifts, hot lapps, etc.
There are classes for all-out set the bar higher each pass, develop better racing technology, and develop durability: Super Street, Prostreet, Prostock, & Top Diesel. These classes will always be driven by the finances of the race team as far as ET Records, HP #'s etc.