Again, Apple already has a super thin ultrabook if you want it thin and light it's the rMB. Makes no sense watsoever IMHO to make both lines of portables thin light and underpowered...
The 12" MacBook is not powerful enough for any "pro" use, however you want to define that. The MacBook Pro indeed is powerful enough. Again, I don't know your specific use case, but I work at an ad agency as a full time graphic designer, and all of my colleagues are totally happy using their 15" MBPs. Many of them thought 16 GB RAM might be a bottleneck (coming from 64 GB iMacs, mostly), but I've yet to hear a single complain about the power of these notebooks.
Obviously Apple builds the MBP line for the 99%. And I am indeed part of the 99%. Because it handles all common graphic design tasks with ease. And like I said earlier, if a rendering or something similarly power hungry is to be done, it is sent to a render farm via the network anyway - not even a high-end 5k iMac can handle some renderings in an acceptable time, so no reason to think a notebook should be able to.