If the system is big enough, then I'd always create a dedicated root aggregate. Does this make any difference if it's 32bit or 64bit? Currently I see no real advantage either way, you're not gaining any benefit from going 64bit. That being said, it may future proof the system to go 64bit on the root aggregate, and as there's now no technical limitations on doing this, I'd probably go down that route.
If there system is a 2000 series of any variety, you simply don't have the storage to dedicate a root aggregate, but the rest of the range the benefits are worth considering. It's similar to aggregate snapshots, you may never use them, but when you do, you'll be so thankful you have them!!!