Dutch Defense- Bb7

I'm trying to learn to play the Dutch defense, and trying to learn the Lenningrad vatiation:

I've heard that the light square bishop fianchetto is to prevent black from putting his bishop on b7... so here's the question. What are black's reasons for doing a fianchetto on b7 when white does not, and he plays non-mainline moves like nf3, e3, c4, Bc4, Nc6 etc.

when fischer played the dutch he never played bb7 so it must be bad

that is so cool dude

I know Alekhine played the queenside fianchetto in the Dutch.  However, the plan seems too slow to me and white can force a weakening of the e6 pawn via d5 then dxe6-dxe6, not that great for the pawn skeleton especially since ...f5 weakens e6 already. 

Ya I didn't see any record of Fischer ever playing the dutch... Dunno what that's about.

Really? It usually seems to me that an e4 break wouldn't be very good for white even without the bishop on b7... Blacks f file opens, and he gets 2 center pawns to 1- just as a generalization, barring specific piece configurations.