So the debate point against is simply that X360/PS3 owners will already buy it on their respective console?
What about Wii U owners that don't have a PS3/X360?
What about Wii U owners that do have a PS3/X360 but want the enhanced experience found on the Wii U version?
What about the fact that Ninja Gaiden 3, Metro :Last Light, Aliens: Colonial Marines, Batman: Arkham City, Battlefield 3, Darksiders II and others are also in development by 3rd parties? Are you just going to claim that 3rd parties shouldn't bother because they are already coming/on the PS3/X360?
What about the easy ROI factor? 250k units would likely cover the porting costs. Anything above breaking even is cost amortization which every developer would love to do.
The rEVOLution is not being televised
