My last company was acquired by a YP, so I know the ins and outs of this stuff.
Each YP is a bit different, so I'm going to generalize a lot here.
The YPs have *lots* of data. If used correctly (and not all do), they can do an amazing job of managing an account.
Unless you're spending a lot of $$, you won't have a dedicated rep at the YP. Support is OK, but rarely great. If you are spending a lot of money - then you probably want a PPC manager instead of a YP.
YPs generally have lots of tracking. Phone calls, contact forms, driving directions, etc. So, you can quickly tell if they are being effective (like anyone - give them a month or three to get it going).
Rarely will a YP migrate your existing account. They generally build a new one from scratch. So, you will lose all of that history you had built up (assuming its good history - if its bad - then this doesn't matter).
Outsourcing your PPC management (to anyone) can save you time. If you don't have the time to optimize your account - then its not just the premium you're paying - you are also paying to get some of your time back.
If you have a really good account, have been managing it for a while, testing ad copy, landing pages, etc, and really understand PPC - then don't outsource to a YP. If you outsource - find a good PPC manager.
If you have an account that you barely touch, it just sits there and collects some clicks and you're hoping it does well, then hiring someone is usually your best bet to increase your returns.