There ya go, that's my answer right there. Why buy a 9mm because the ammo is "cheap" and then pay out the backside for +p? Now you've got ammo that costs more than other calibers, negates any perceived advantage of reduced recoil, is more difficult to find and is harder on the gun. Or.... you could just buy a 357sig and be done with it. There's always a cartridge that can be loaded to standard levels and do the job of +p "lesser" cartridges.
Unless somebody shoots a 10mm +p+, then you might be hard to beat.
Peetzakilla, I disagree with this statement. 9mm ammo is about as cheap as can be had for pistol ammo short of a .22; I can still buy it at $190/1000 locally, and reload it for half of that. There's no way you're going to find .357 sig ammo even close to the prices and quantity 9mm can be found.
My carry load has always been the same for 9mm; Federal 9bple. I have traditionally bought and continue to buy them at $25/50. I suspect .357 sig 'defense' ammo is at least double that, and is considerably harder to find than typical 9mm defense loads on top of it all, not the other way around as you state.
As has been mentioned before, 9mm service autos are largely built to handle 9mm NATO, which has pressures akin to commercial +P+ actually. So, you're not being any harder on the pistol than it was designed to handle; as I mentioned before a local department runs the same load as me in their 9mm Glocks, and according to the armorer have not experienced any side effects due to the ammo at all, despite the many years the load has been issued.
Recoil of a 9mm +P+ is percieved by me be less than a .357 Sig. I imagine it would be percieved by most people with most pistols that way, so I don't buy the recoil argument either.
As far as power goes, heres a comparison from Federal law enforcement ammo:
P357SHST1 125 gr. @ 1360 FPS 513 Ft-lb
9BPLE 115 gr. @ 1300 FPS 432 Ft-lb
Is 80 Ft-lb really worth the extra expense, recoil, hassle to find ammo, etc? For me it wasn't. If the Sig round had performed as originally intended and launched a 125 gr. slug at 1450 FPS, a la .357 Mag, then I might feel differently; but right now I feel it's really just a warmed over and relabeled .38 Super, and that round has been all but abandoned because of the 9mm +P+ loads.