While they seem like a good idea, and may be useful in a "clean" environment (police work, range use, or a fixed position), they aren't suitable for military-type field work.

One of the problems is that the ammo in the spare mag tends to work its way forward under recoil, preventing you from being able to insert the second mag when the time comes.

Another is that the expose mag tends to collect dust, mud, and/or sand.  This will not lead to a pleasant shooting experience.

Third: weight.  It CAN be an issue.

Forth: a duplexed mag setup either won't fit, or takes up a lot of room in a mag pouch.  You pretty much HAVE to drop them and leave them, which can be tactically unsound.

Fifth: it's a pain to have the empty mag hanging off your gun when you're running around with the second mag half-empty.

While you can work to solve these problems, doing so is usually more difficult than just reloading with standard single mags.  With only a little bit of practice, you can reload from your mag pouch within half a second of someone loading from their second, connected mag.

I'm not saying: don't use them.  Just make sure you use them when it makes sense to do so, and don't use them when it doesn't.

-Troy