I don't see any reason to disallow them on SC, so I vote yes because, why not? Assuming, of course, they would not be available the first few weeks after ladder reset. That being said, after reading other replies, I still do not see them as very useful, so I would not buy them.
But for Hardcore? Unequivocally NO, on principle. Hardcore isn't suppose to be easy, the levels aren't supposed to be free. The entire point of Hardcore is to gain levels without dying. Levels are THE reward, THE mark of success, the entire point of hardcore. Being able to skip even a single one defeats the entire purpose.