From what I've read, if you plan to keep money in your PayPal account, I see nothing to lose. There isn't a minimum balance or any additional fees. Personally, I rarely keep money in my PayPal account. If I sell something, the money usually gets spent pretty quickly on more eBay purchases. So, with my account balance at zero, when I pay for something, PayPal debits my checking account directly to pay for it.

However, the money market option looks like a good one if you're going to leave some funds parked there.