I used to clamp two mags together for my HD carbine, granted this is an M4. My thinking behind this was so I could load 20 rounds into each 30 rounder and leave them loaded for a very extendec period of time without damaging feed lips ect, plus this would give me 13 extra rounds over my typical mag loadout of a singe 27 round mag.
I discontinued this because the mag not in the mag well covered my bolt catch and when I switched mags the empty would cover my ejection port. Plus it adds more weight and bulk to a rifle i need to be as light and as bast handling as possible.
The only time I would consider doing this again is for a stationary defensive carbine. I would never utilize a clamp for a patrol rifle especially with aluminum USGI mags. Too much chance of tweaking the mags and causing a malfunction.