Go for it if it makes you happy I get different color/material of the same slg to match with other slgs. I just like what I like. If it's silly, then put me down as a happy member of that silly club...lol!
I honestly do not like having more than one wallet. The thought of having to change out not only my bags but my wallet too is just not appealing to me at all. If you do not mind that then go ahead, why not?
My Chanel SA told me that sticking with one wallet is best. That was her opinion, and I found it interesting that she wasn't trying to hype sales of wallets. But I do see her point....you can just move your one wallet from one bag to the next. Keeps things easy, though it wouldn't work for women who want their slg/bag brands to "match" or want to color-coordinate wallets with each bag. I have 2 Empreinte leather wallets -- one for Spring/Summer, one for Fall/Winter. I also have a Chanel wallet, because I consider that a classic staple item and it matches my flap. Since I'm not switching wallets more than once every few months, it works for me. If you want the Mono wallet, and the money wouldn't otherwise be going toward a new bag instead, go for it!
I have quite a few different wallets, card holders etc, but not only LV. I really need different sizes as I have very small and very big bags.
But I am also a bit of a collector of pretty SLGs .
So if there is a wallet that your really like, why not go for it?