I know they're individual purchases, I was just having a debate with myself regarding whether I want to go all in on PC and never touch the PS3 version again. I've already greenlit.It's the same you'd do if you switch to any other platform that doesn't share the stores you own it on. PC/Mac/iOS/PSN/XBLA/Android are all unique purchases, we've known that from day 1.
All you really have to do is vote to approve it on Greenlight so the chances of it getting on Steam go up, and FarSight will get more customers which helps them get more money and do more/bigger tables. That doesn't cost anything, and helps out indirectly even if you have no interest in buying anything.