I actually end up rushing to get things done for a convention simply because of the other stuff I have going on in my life (AP biology plus figure skating plus art equals no life at all). I might come up with a really, really good idea two days before a con. Getting it drawn and printed before then? No problem. However, if I had to make an original art piece to go along with it, I don't think that it would be very manageable. Therefore, in some incidences, the 50/50 rule would definitely be an impairment.
Honestly, in any business-ish environment, there will be people who use some not-so-nice methods, such as tracing fanart, to get ahead. If these people put the traced are online, it can be caught, but there will always be some people who disobey the rules, and they won't always be caught immediately. I think that trying to enforce tons of regulations, such as the 50/50 rule would not solve these problems. There will always be sneaky people, no matter what. I understand that the convention can get into legal trouble, but I don't think the 50/50 rule will do anything to help. In fact, it could actually make things worse. For example, some desperate person might think, "I don't have enough original art... why don't I go find some nice obscure original art on the internet and change it a little".
I agree with princeofrose. Forcing me to sell original work would be stifling, not encouraging. I generally don't feel like scanning and coloring my original art. Most of it stays in my sketchbook and is colored with Copics, if at all. However, the vast majority of my fanart is digital. Basically, I have a completely different creative process depending on what I want to draw. It doesn't mean that I value my original art less; I just like to keep it in my sketchbook. If I was required to sell original art, I would almost HAVE to make it into full color digital art in order to make it presentable. I would do this if I had to, but at the moment, I'm just not that interested at selling my original art, and I do think that, if the 50/50 rule was in place, I would most likely do a bunch of kinda scribble-y original art postcards (the easy way out). I don't think that's very creative, but with fanime in less than 90 days, it's what I would have time to do. Therefore, I think the 50/50 rule could definitely detract from the quality and creativeness of the art sold at fanime.