There's really not that much to say about Sebastian Rincon, unfortunately, as he's seen very few minutes this year with the first team and zero minutes during an actual MLS regular season game. One of the reasons for that is probably because he's on loan from Club Atletico Atenas of the Uruguayan first division. So despite his potential, it might just not have been worth the Portland Timbers' time to play him rather than some of the players they actually owned.
That;s not to say that Rincon doens't have potential. I think it would be difficult to say that most fans wouldn't want to see more of him based on some of his performances in the reserves (and against Aston Villa). Not to mention, of course, that he's still a very young striker with a lot of potential. Developed correctly, Rincon could become a great MLS striker one day, but not at the pace of play that the Timbers seem to play him at.
Should he be kept for 2013?
Well, first off, it might not be worth it to either Club Atletico Atenas or the Portland Timbers to keep him here. This would be a far easier question to answer if his contract was strictly owned by the Timbers. As such it comes down to a matter of variables. Will Club Atletico sell him? How much would he cost? Is it viable for the Timbers to have yet another young striker with potential on the roster?
In the end, it's my opinion, that if the Timbers can acquire him for cheap then he would make a valuable addition to the squad over the coming years. However, if it's just going to be for another loan year then it's probably best to use that money and international roster spot on a player who the Timbers can keep permanently.