I think it depends how many mags you get through. I've done a fair bit of practice at reloading and always run pouches with quick access, also only tend to use one or two mags between breaks so that stops the rattling as you run. Never got anywhere close to running out of space in my dump pouch, most I've had in one is 2 rifle and 2 pistol mags but that's a rarity, usually just one rifle mag. If they're plastic mags of any kind (which probably covers more than half of all airsofters loadouts at a vague guess) the problem is lessened further. I've been noticing recently they don't suit GBBRs so well, don't want much more than 2 GBB M4 mags swinging around in there.
Again though depends on the user, I'll probably only reload once most games so having 1 mag in the dumper is ok and you can always stow empty mags securely in a pouch during a lull, indeed you probably should. Generally in airsoft there's plenty of opportunities for doing that. In which case the dumper almost purely acts as an aid to make reloads that much faster.