One of the main adavantages is reducing flats.
I don't think it's such a hassle converting rims to tubeless. I just converted mine with this kit: http://www.sincamaras.com/INGLES/indexenglish.HTM. I know there are several other on the market but my local bike shop recommended me this one. I decided to install it because in my area, this winter, a lot of vegetation has been cut around the trails and everyone is having lots of flats due to the spikes on the ground. With the sealant liquid I hope to hugely reduce the number of flats I got last months.
9th July 2012 update: After almost a year and a half with this system I can say I just got one flat and it didn't affect my ride, I found it later at home.
Another important advantage of removing the tubes is making wheels lighter.