Surely music in horror games is as important as the game needs it to be.
The 'less music, more sound-effects' trend (if it is even a trend?) may just be people realising that literally every thing you hear in a horror experience (game or film) is important to the creation of a specific effect the creator is going for.
I think one of the most disturbing sounds in any horror game is in Amnesia/Dark Descent - the clicking/grinding sound as sanity depletes.
For me, cohesive sound design overall is the secret to a great, oppressive or horrific atmosphere. Music in various forms is just another part of that. S.T.A.L.K.E.R.'s an incredibly atmospheric and unsettling game, from the ambient sound beds, the effects (dog angry... dog running away! dog angry again... ), and even the fellow Stalkers huddled miserably about small fires, picking out melancholic tunes on their battered acoustic guitars. Everything reinforces the tone of a bleak, hopeless, hostile world (Metro 2033 does a similarly effective job. Russian/Ukrainian post-apocalyptica > Western, I reckon!). And when you head down into the nastiest areas of the Zone, the horror soundscape dials it up a notch to suit the increasingly supernatural ambience (and threats).