Pretty straight-forward- what're the benefits and drawbacks of dry or liquid yeast?
As Jeff said, the biggest advantage of liquid yeast is variety. Wyeast and White Labs offer a huge variety of yeast, including seasonal and limited time offerings.
One thing to take into consideration when using liquid yeast is the number of yeast cells. As John Palmer mentions in his book "How to Brew" dry yeast contains more cells than liquid yeast and more cells potentially = better final product. For that reason, he recommends making a yeast starter when using liquid (dry doesn't even have to be re-hydrated first). However, he states on his website (not in his book, at least not my copy) that they have been coming out with larger 175 ml smack-packs that do not require a starter.
I think the main thing is variety. There are so many liquid yeasts to choose from, and they'll let you produce a much larger variety of beers compared to what you could produce if you just stayed with dry yeasts.
The advantage of dry is they're easier to store and they'll keep longer. Having a few packets on hand is probably a good thing, even if you regularly use liquid. If you make a batch of beer and realize you forgot to get yeast, or if something goes wrong with your yeast, a dry yeast stored in the freezer could save the day -- espicially if you don't have a homebrew supply shop nearby.