That's a very good point, and I think you would want a softer wheel, yes, and probably white (which I think the off-ice wheels are) so it didn't mark as much. Softer wheels are grippier, which would be good on a less abrasive surface indoors, and wouldn't be inclined to wear down as much indoors. I think if they are used outdoors they will wear more quickly. Lloyd Jones posted on YouTube that his off ice wheels when used outdoors got destroyed pretty quickly - of course he would be harder on them anyway. I guess it's an occupational hazard. There are smaller wheels out there that are harder that might fit an off ice blade.
I doubt I'll be able to do much skating on the ice in the future, but am happy enough with rockered slalom inlines - they are different, but not inferior, IMO.