[Phalanx]

Depends on the type of film...if it's one that's clearly out to terrify/horrify from the get-go, then throwing in a joke or two probably isn't the best idea. The Thing, 28DL, Candyman, Halloween (the first one anyways), Hellraiser...seem as though they'd have lost a little of their atmosphere had humour been in abundance..
However, if it's the kind of film that's clearly not meant to be take all that seriously, things like "jason goes to hell", shaun of the dead, Slither, From dusk til Dawn, evil dead, idle hands...a few one liners/jokes between the characters and scenes that make you laugh along with a cringe are quite acceptable to me. Whether or not "comedy" is the whole feel of the film, or just a throw in, I think that for these kid of films, there's no problem. Sometimes works well to get unexpected jump factor in there.
It's too bad they turned Freddy into a comedian after the first couple of films...had they left that alone, he probably would have been remembered as one of the most menacing villains ever...
Seomtimes comedy works, sometimes it doesn't, I think it's more a matter of knowing what type of film you're in the mood to experience, and going with that. If you don't get what you want, you're aware of it for future reference.
In short, no huge problems with the comedy thing here, as long as it's expected along with the kind of film it is...and yes, I agree - I'd be bothered if something was billed as "spine tingling/absolutely horrifying...etc" and advertised solely as such, then it was full of laughs...