Yes, coffee at full strength works very well for some species (namely the ones growing in ultramafic soils) and not so well on others. And although it does contain nutrients, you're basically re-acidifying the media, which is much more important than most growers seem to understand. For the species that don't seem to appreciate it, you can see an obvious difference in where they grow, such as limestone, sand, etc., which supports the real benefit of coffee.
As an aside, a few "natives" I've spoken to say that all the Nepenthes they've in the wild are full of ants, so much so that the liquid in the pitchers looks like an "ant porridge." Could be something to consider..