Thanks for the replies guys. Makes a lot of sense!

I think there are two major reasons for ...h6. One, as mentioned above, is the need to think about a forking attack on h7, typically with Qc2 pushing down the c-file while a bishop is already on d3. The other reason is not found in opening books because it's just categorized as a blunder when it happens. White can make blunt combinations using threats to capture on f6 next move, removing the guard of d5, h7 and e4 or displacing an essential piece, and Black cannot pull the emergency brake with ...g5. Such combinations are no threat after ...Nbd7, which explains why ...h6 and ...Nbd7 are so seldom combined (notwithstanding the very recent fashion for trying this), e.g. the Orthodox and Cambridge Springs do not feature ...h6.

Admittedly against weaker players, or in faster time controls, or if you're lucky, these will not become an issue and you might win a tempo.

Is there any drawback to playing 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Bg5 Be7 5.e3 0-0 6.Nf3 b6? I don't think I've ever come across this line before without ...h6, and I was wondering if there was a problem with it.