I can see them being useful only if you use the "beer can" style grip to reload, as you can get much more magazine in your hand, as peaches said. They also hold the magazine in tightly, so one doesn't have to worry about them falling out, and can even mount them upside-down (especially in the case of airsoft magazines, as they're considerably lighter than real ones). I saw some of the
Ebaybanned copies at End of Lies, and I was impressed. I will probably soon get around 7 of them for my CQC loadout, as I do use the beer can style grip to reload.
They are also lighter, and somewhat less conspicuous, that normal mag puoches, so if you[url] were attempting a lighter or somewhat 'concealed" loadout, you could throw a few of these on a belt, a KAC PDW under your shoulder, concealable armor, and a stonking great big coat over it all.