As far as taking gear on/off the belt it depends on the match. I did strip a bunch of my gear off my belt at the last TH3G match because we finished on the urban prone stage and stripping off the gear I didn't need let me get down lower to the ground. Most matches you will pretty much configure your gear before the match and leave it that way the entire match, not because of the rules but because you'll need it all and won't have time to keep reconfiguring everything.
That being said, modular gear is an advantage if you compete in lots of different types of matches. I use the same belt for IPSC handgun, IPSC Shotgun, IPSC Tournament/Grand Tournament, and 3Gun, and depending on the match I configure things differently. E.g. for the IPSC shotgun provincials all I had on the belt were shotshell 4x4 caddies, and I ran 1 more caddy than usual. The next morning I reconfigured the belt pretty quickly upon arriving at the TH3G handgun/shotgun match to take off one shotgun caddy and put on my pistol pouches and holster. If all your gear has quick on/off features like the Bladetech Tek-lok attachment or similar then it's faster and easier to change things up without having to take everything off the belt in order to slide on one more pouch in the middle.