Why not?
As npmjs.com states: npm is the package manager for [...]
Package management made ezpz!
I use npm as my build tool as well. After configuration nightmares with Grunt I initially wanted to try out Gulp. While poking around in the web for best practices I found a blogpost about not to use gulp or grunt or any other of this kind. Challenge accepted! Why not use npm if it is capable of doing all the things Grunt did for me.