That's an interesting question - what do we want in Leo? I'm also sometimes rather amused by awful suggested translations by natives and non-natives alike, but am also regularly amazed (and depressed) by extremely good translations by non-natives. It must be hard for people posting for the first time to decide whether or not a suggested translation is good, if they are not that fluent in the target language. When you've been around Leo for a while you do start to get an idea of people whose suggestions are especially good or those who are just trying to wind people up.
But how could we prevent people from posting when they haven't got much idea? Even on http://www.proz.com
, where there is a screening process of sorts, people still make silly mistakes from time to time, claiming to be "very sure". It's true that on Proz you can see what the person claims is their background by clicking on a button. But you still have to be fluent to get the whole picture, and the extra "security" can be cumbersome.
I prefer Leo's pleasant atmosphere and the fact that anyone at all can make a suggestion. When someone does suggest a really bad translation, people usually explain the mistake in a fairly polite manner and give the reasoning behind a better answer. As long as the person with the question doesn't just accept the first answer, and reads others' commments, usually he or she will end up with both a decent translation and a better understanding of the whole problem.