To me, it is like trying to get down to one tool in my toolbox. To a man with only a hammer, everything is a nail. My .22LR is for something different than my .308 is for.

A good tool kit has both SAE (.22 .38 .357 .44 .45 .50) and metric (9mm 10mm) wrenches. Sometimes you need a small (20 gauge) or a large (12 gauge) slide hammer. And a wrench will not substitute for a screwdriver so you need a few sizes of them (5.56 7.62x51 7.62x39).

A thing should be made as simple as possible, but no simpler.