I think it's mostly been said already, The way I analyse things like this is either
A. You don't know who you'll be facing and there are many armies where it just won't help at all, or
B. You know who you'll be facing and are sort of unfairly taking advantage of the fact taht you know which army you'll be playing (I feel this way a lot when my DH friend takes Sacred Incense when I play Chaos

( it's not like I can tell him not to, because it's a valid wargear selection, I just think hate when he does - playing friendly is supposed to be about having fun, not winning every time)
I of course don't mean to imply that you're like a bad person or anything, this is just how i think about issues like this, hope it helps