Honeycut summed it up nicely. If you got protools, the 8 outs wouldn't be totally necessary, it'd be more of a luxury.
Without the output board, you can record
one track/sound
at a time to PT. Then you can then EQ and furthur edit each individual track to get a good mix.
The 8 outs is valuable to someone like me because I don't record to a multitracker which lets me edit each individual track after I record it. I record to a CD recorder which recieves a stereo signal (L and R channels) from my mackie board. So to have the 8 outs means I can EQ and pan individual sounds on the mixer before it gets recorded.
I don't have the output board. I've had my XL for over a year and have been recording my stuff for a couple months. So even in my case, you can manage without it (but the proof is in the pudding I guess!

). I'm looking at buying it now, so it should be good. But if I were you, getting protools and all, I'd save the money and maybe look at getting it down the track.
Peace