well, should the unthinkable happen when you are trying to use the pistol under stress, and the mag release gets bumped, OR you fail to shove it fully home, but have gotten it close and dropped the slide, THEN it falls free, you'd still have at LEAST that 1 round usable...
it is a bit far fetched, BUT not outta the realm of reason!
FWIW, I once had a firearms instructor show the class, with my UNALTERED smitty that it couldn't go bang with the mag dropped...
THEN I stepped to the line, racked one in, held back the trigger just a BIT, dropped the mag LOUDLY onto the table, and then let fly... BOY was he surprised! (and he made me show him that trick)