That way you could buy it with all of the necessary motor, battery, and esc mounts. Plus it could have some sort of center diff capable tranny, and maybe a 2speed that would still allow motor brakes and reverse.
I would prefer the kit, as I like to build stuff, but the roller would also be cool. That way any motor/esc combo could be used. Traxxas could even use a plastic center chassis with the trays, etc molded in. Sounds good to me.
While they are at it, they could offer the emaxx as a roller too. Knock of 150 bucks for the radio gear and brushed motor/esc. Those would sell too.