I think the appeal of using cellphone touch screens as controllers is the relative availability of tablet style phones and their inexpensiveness.
The RP controller is expensive (And rightfully so, I've no issue with the cost as it is), but almost everyone has a cellphone, and those that do likely have outdated ones laying around not doing anything. I know myself, I've got 2 old iPhones sitting around here not being used. That "repurposing" of an old iPhone as a controller for my RP system would be a great thing, because I could add operators to my small layout--right now there's just one controller. Yes, I understand they wouldn't be as easily used as the RP controller, that's not the point-But it'd be nice to have a second or third controller for the trains with minimal expense while I work towards expanding my RP control system.
I mean, RP has the CI-1 interface, and that in theory turns you computer into a controller as well, I'd think they would be able to make something like that happen.
I say the "in theory" part above because I'm using mac's and the old old Dell laptop that got retired at work needs to be upgraded to even use with the RP ci-1 interface system.