I have a redi-mag on one of my "go-to" carbines, it doesn't couple the mags but allows the carry of a spare. I think it is better than a coupler in case you have a malfunction and have to quickly drop that mag to clear it.
Never went that route personally, a pocket or mag pouch works more than fine for me. I don't like the added weight and it really does throw off my balance with the weapon. Mike has all the good toys - show off - but I'd consider a stock mag pouch first, just my ever so humble opinion.