Not that I use a money clip - I have a range bag, er... purse.
One thing that slightly bugs me about people who pocket carry, like Oleg: he's got his wallet in his back pocket (obvious wallet imprint), and a pocket holster for his P32 in the front pocket (also makes a wallet imprint). It looks like he's got two wallets. Not that alot people these days are that observant, but it does look strange.
Tactically thinking, I figure a money clip eliminates the extra bulk of a wallet and breaks up the squarish outline? The more low-profile nature may prevent pickpockets from getting tempted. It also seems some wallets are big enough to prevent buttoning the dress pants pocket it's in, but a money clip may not, allowing "proper retention".
I also figure a money clip helps persuade the owner to carry less "stuff" in it, since a clip is limited to the amount it can sufficiently hold. That's why I try not carry a huge purse, I just get lazy and stuff more useless gunk in it and not clean it out.