I can and have done 10g, but I really like variety. I don't have a lot of friends that drink "good" beer, so I drink about 80% of what I brew. And since I brew 3-4 times a month...ten gallons is a bit much.
I do brew ten gallons when I'm brewing something that co-workers/friends would like (heffe, pilsner, etc).
It's also nice to have the setup for high-gravity brews. I can collect 12 gallons of wort, and boil down to 6 for fermenting.