Unless we know the specifics of the injury, and at this point, that seems very murky at best, we just don't know if playing on it will risk further damage or not. In McHale's case, in the oft-stated yet way off comparison to 1987, it was known he had a stress fracture in his foot, and he knew he was probably doing permanent damage by playing. Although he still limps, he has said all along that it was worth it, the C's came within 1 game of winning it all, and never made it back to the Finals again until last year. When it is in front of you, it is hard to play for next year, you just never know if you will get close again, and McHale never did.
With Garnett, unless the C's come out and admit they were lying and that the MRI did show ligament/structural damage that needs reconstructive surgery, something they have vehemently denied up to this point, it is not at all clear, that besides his discomfort and effectiveness, that playing will do permanent harm. If it is indeed a severe muscle strain, which can take much longer than most people think to heal, and the swelling won't go down in time to let him be 100% for the playoffs, it could just be a matter of dealing with the discomfort or not.
If in fact they already knew it was the type of injury that would surely get worse and could be career-altering, I find it very curious they would string this along, only to rule him out for the year at some later date because of risk of further injury. I would be very disappointed if that were true because it would make me very cynical about their motivations in terms of selling playoff tickets, season ticket renewals, etc. As much as I don't want KG to go down, if it has really been more serious all along then has been let on, I would have preferred they just shut him down, get the proper surgery, and do the best they could the rest of this year. If all of this drama was just to keep hope alive in the fan base to keep sales strong, I will have lost a lot of respect for the C's organization.
Look, I don't want to turn you into a cynical person - if you've lived so far without being one, this is not a good enough reason to change the tune, but Ainge has just admitted there's the possibility of a surgery down the road. I think that pretty much rules out the muscle strain possibility, no matter how severe it could be, no?
Personally, and although I don't think of myself as a cynic (not that there's anything wrong with being one), my first comment when they announced the muscle strain was to put the hypothesis they were hiding something. I don't think this is necessarily ill-intended - it's just a way to protect the player, his teammates and the franchise. I'm also fairly certain they haven't ruled out KG for the year, at least yet (and I don't think that will happen).