Quoted:
I am surprised nobody has mentioned my favorite reason for not using them:
When the two magazines are even in height, you swap from the right magazine to the left usually (as viewed from behind the stock), now there is a magazine right next to the ejection port. Murpy's Law will bite you at the most inopportune moment like a perfect string in competition, the chance to qualify expert or worse, he will bounce that piece of brass back in the ejection port when you are in the middle of a firefight...
hence the reason for the staggered setups now on the market.
i like brownell’s coupler for 4 reasons:
1 - the mags are staggered
2 - there’s a top round protector for the second mag preventing any movement of the top round (this feature also doubles to help keep debris out of the mag.)
3 - they’re inexpensive
4 - they flat out work!
i’ll take the extra bulk and added weight for the advantage of having an additional 30 rounds on standby. in a pinch i can just grab the rifle and go, knowing that i have 60 rounds at my disposal. this is what works for me, obviously YMMV.