Is it a bad idea to submit a paper merely to elicit reviewers' comments?
The other answers mention that submitting a paper only to get some feedback is a waste of time for the reviewers, and it can be particularly damageable for your reputation when the submission is not double-blind. Remember that academia is a small world, and reputation a very important aspect. 

In addition, submitting a paper to a venue usually prevents to submit it to other venues, at least during the reviewing process. When you're pretty sure that the paper will be rejected, you might miss other opportunities. 

Finally, there is another risk: the paper might be accepted! Of course, at first, you might think it's a good thing, but if the results are not good enough, it might hurt your reputation, and there is of course the possibility that you're wrong. Publishing wrong results (and I'm only talking about mistake, not fraud) is far worse than getting a paper rejected. On top of that, if the paper is published, it might be harder to publish only the improved results, as they might not consist a novel contribution on their own.