There are a great number of corruptions that I wouldn't keep voluntarily. Stiff Muscles is one of them, no question about it. Grey's suggestion isn't based on the consideration that players would universally remove it right away if they could, but that it's lethally bad. As in, characters who get stuck with it are going to either remove it quickly or be sufficiently crippled that they cannot handle the rest of the game, period. I don't think it's quite that bad.
Some corruptions in the pool are overall good. Since corruption is supposed to be a "playing with fire" sort of gamble - bartering with the forces of chaos for blessings that are curses in disguise - some corruptions in the pool should be overall bad. The only question is whether or not stiff muscles, as it stands, is too bad, and what should be done to remedy it. I just happen to think that reducing the energy cost penalty - to less than half of what it was no less - reduces the "I'm screwed now" response corruptions are supposed to elicit. Increasing the boosts not only makes the character more likely to survive through the immediate aftermath of the corruption should you get it in the midst of battle, it also does a better job of disguising itself as a blessing, when in truth it's still a terrible corruption that you should remove. A corruption like "Your feet have transformed to hooves" barely elicits any emotional response at all. "Fuck I'm screwed" is much preferable to "meh" when it comes to corruption.
(Though with stuff like +8 St, +6 PV, +4 To I can even think of some situations where characters would survive thanks to the boosts; for instance a fairly anemic early game PC caught in a mass of chaos rats, going from being nibbled to death to being impervious to damage.)