Why do you have to justify at all? As long as its your money that you are earning, spend it however you choose...but if it's your wife's money that you are spending that is a different story.
If it is your money and she is complaining tell her to STFU.
Consider that women spend like $500 on shoes and handbags and crap, what's the big deal if you spend a few bucks on your passion? I would tell her to STFU if she complains.
You've never been married have you? Or is it that you have...multiple times?

Here's how we work it, we each bring money to the table. We have divided up the bills so we each pay about the same amount for bills (even though she makes more than I do.) If we're saving for something specific, then leftover will go towards that, if not, then we can use our own money as we see fit. I don't complain about her buying scrapbooking stuff, or clothes, or whatever she's buying. When I come home and have a new toy and she says, "what's that?" and I say, "a new bass" and she says, "don't you already have like 3?" and I say, "yeah...but this one's different," she just says, "ok."

