They used to have a list of approved holsters. It didn't work very well. Do you know how many holster manufacturers there are, or how many different models and styles there are? Lots of pefectly good holsters couldn't be used just because they weren't on the list. Plus, supposedly IDPA was spending lots and lots of time looking at all the different holsters people wanted to use to see if they should be put on the list.
The point is not trying to see light between the holster and the wearer while concealed (of course you'd never see light if it was concealed), but the intent to come up with a general rule of thumb that would differentiate between "practical" holsters and gaming holsters. And it's not if you can see ANY light, but it really means if there is a significant gap between the holster and the wearer.
Yes, according to the letter of the rule, many non-gaming holsters were also made illegal, but it did stop the use of some of the most non-practical ones. Even at the S&W IDPA Winter Chamionship they didn't try to literally enforce this rule. As long as the holster is not obviously a competition-specific model, noone will complain.
IDPA is full of "silly" rules, but it's their game and if you want to play you have to follow them.