For me personally, not having to run my plugins in a bridge when I run my DAW in 64bit mode. Also, I suppose if you have a ton of plugins and for some god forsaken reason you haven't tapped out your CPU load you could get them all loaded at the same time with the DAW as long as you have enough RAM and, of course the CPU power to do so.
That's a tall order though. I've got a quad core and even with that I doubt I could load enough AUs to tap out 4gb of RAM before the CPU would spike. So it's really just the convenience of not needed the bridge, which, fast as they may be, adds a certain amount of latency in my experience.