1) Self-discipline is virtuous in and of itself. We shouldn't develop alternative ways to control weight and/or maintain muscle tone without controlling food intake and exercise, because that undermines the tangible rewards of virtue.
2) Biology is extremely complicated, and the drug that keeps you compact and muscular in your 40s might end up promoting the growth of cancerous tumors all over your body in your 60s. (As happened to the older mice who received high doses of GW501516.)
I don't agree at all with the first argument. The second one is alarming enough that I'm not going to be the first or even the millionth person in line to obtain these compounds through grey-market channels and dose myself. Similarly, I would love a drug cocktail that prevents the unwanted physical effects of aging, but I'll let other people blaze a trail with human trials (clinical or self-experimentation.)