Perhaps I should have said IDPA and USPSA don't allow "high cap" revolvers to be used as designed which would give them a clear competitive advantage. They are not totally banned.
By the letter of the rules, you can use a 7 or 8 shooter in IDPA, loaded with six. No problem for the first six properly indexed rounds. What'cha going to do on the reload? Leave a gap in your speedloader and take time to index it?
By the letter of USPSA rules, you can use a 7 or 8 shooter, you can even load it full, but you just can't SHOOT but six before reloading. Can you count to six and stop every time?
There is some discussion of allowing 8 shooters in USPSA Revolver at Minor power factor, only 6 shooters at Major. One competitor shot around twice both ways, counting his 8 shot as Production, and found the 8 x Minor to be a clear winner over 6 x Major.