I put the same things in my minis as I do in my wristlets - anything I need fast access to - cash, change, driver's license, debit card and "club cards" I use frequently.
I pretty much rotate my accessories with my bags, so I wouldn't say I use one more than the other, but I find both wristlets and skinnies to be much more useful than I first imagined them to be. I still carry a wallet separately b/c I just have too darn many cards (not that I use them, mind you)!