I mean what are his powers for if he does not use them? :P

 Most of the time he does any drastic healing it results in Josh falling into a coma state for awhile, but techicly he could heal the xmen of preety much anything. But he would have to go in critical order since theres no telling how long before he can heal the next person in the line.

Because he's still on Genosha, helping to tidy up the mess left by Selene.  He can't heal the wounded X-Men from half-way around the world.

" His powers are on the fritz after killing Wither, I think. "

but... isn't he looking so GOLD and WELL in New Mutants #13? I mean there was a cameo of him there (in the mess hall) eating with the rest of the mutants. :P

He's having some trouble with his powers due to his recent black exploits (killing wither and putting a tumour in vanisher etc.)

Excuse to kill off the X-men of course.

His powers aren't 'for' anything, they're a genetic mutation that he just happens to have been born with. Also, you'll notice that healing does tax him, both physically and emotionally, so it's not like he's an unlimited resource.

Plus it takes energy for him to heal. He could be forever healing minor injuries but bigger injuries seriously drain him and even endanger his life. Sort of like Scarlet Witch back in the day her powers drained her and exhausted her. Course now she manipulates reality on a whim so who knows.