I was surprised about the Roomba too. Maybe it doesn't look or act enough like prey. I have a basket of various cat toys so they can go pick out what they want to play with. I find them all over the house then bring them back to the basket.
I brush both of my cats daily because they enjoy it, but very little fur comes out when I brush KiKi. Tigger does shed, so I brush him also for de-shedding.