Why not? Let all the regions of the UK have more power over this sort of thing, like they do in the US, Germany, Spain, et cetera. Not to say take all tax raising powers from Westminster, but certainly some. At the very least it would encourage a bit more fiscal responsibility in local governments.
Without a doubt it is more democratic. If the Northern Irish wanted different taxes to the rest of the UK, why should the rest of us be able to gang up on them and stop it? As long as the amount of money they receive from central government is unchanged, not really our business.