See I cut my teeth in AD&D when it took forever to gain a level, and when you encounter a creature that could level drain, there wasn't anything you could do about it- not getting hit isn't really an option (well, I mean, it is an option, if the DM is allowing parries and such, but the first time you realize your opponent is a vampire/wight/spectre/wraith you probably weren't prepared for it).
Losing because you failed a roll is rough. Losing because the DM succeeded at a roll? Blah.
I never used level draining monsters as a DM because it just felt unnecessarily cruel- killing the PC's is usually more merciful!