Because Disk Utility requires you to erase the drive when you partition it. So if you're booted from it and using Disk Utility, it'll be grayed out. You'd have to boot from another volume (like the installer DVD) and use Disk Utility on that to partition your boot drive.. again.. it WILL ERASE EVERYTHING ON THE DRIVE YOU PARTITION.
IMO, there's almost no advantages to it. Because later on when you need to resize your partitions because it's not large enough.. you can't do it very easily. And I don't trust apps that change partition maps on the fly. So I'd be backing up my **** anyway and might as well use Disk Utility to do it right.
There's plenty of threads already in existence discussing the pros and cons of partitioning, hit the search feature and you should be able to find them with a little wading.