The rules on guns at first may seem silly, but they are logical. The idea is to keep types of guns together in same the class. Big bore single-action autos together. Med. bore S/A's together. DA's/DAO's/Glocks together.
Within each class there is some competitive advantage to possibly be had, if that's your thing. In SSP, for ex., the 9mm Glock shooters may have an advantage over .40 shooters, due to the lower power factor of 9mm. It's the shooter's choice, of course.
I personally think the 9mm Glock shooters have an advantage with lower recoil over .45 CDP shooters and their better trigger (not an advantage at most combat distances, imo). The scoring parameters as determined by the IDPA Board doesn't agree with that, it seems.
The nice thing at my club is that people aren't fascists about the rules. Rare tech inspections. If someone wants to "cheat", they'll be discovered soon enough. It's really to their disadvantage anyway. I would say the biggest problem, imo, is people not shooting in the class they should be in. Staying in lower ranks in order to win more. Not much you can do about it, i guess.