Very simple answer WWIIArmsLover: Versatility.
I dont like to load magazines as much as the next guy. They are very handy at the range!
There is some guy on here that has a 32 round clip for his sub. Try concealing that badboy!!
The sleeve on the 15 round clip is really only an inch longer and gives you the capability for a full handle. But if you want to wear it on your ankle put in a 10rd and be done with it.