Is it a bad idea to submit a paper merely to elicit reviewers' comments? Some days ago I was talking with a friend of mine, who is currently a post-doc at the illustrious Italian Institute of Technology (IIT) working on humanoid robotics.

He was quite disgruntled because he recently spent a lot of time testing his latest discoveries and writing a scientific paper to submit to an important artificial intelligence conference, but on the day of the deadline, his supervisor asked him not to submit the paper. Indeed, his supervisor felt the experiments were not good enough, or the results were not excellent.

Anyway, he did not want him to submit the paper, and so he did not do it.

We discussed about it and we both stated that, even if the paper and the discoveries were not excellent, they should have sent the paper anyway. The eventual rejection maybe would have come later, by the reviewers, together with the important review notes.
This way, they would have probably got a paper rejection, but they would have been able to treasure the reviewers' review notes.
Instead, with his supervisor initial rejection, they won't get anything except their opinions.

So we thought that his supervisor had a bad idea.

My question is: why did his supervisor suggest him not to submit the paper? Why did he not think about the possibility of getting at least the reviewers' comments?

Is a paper rejection so grave a fact?
Is it so dishonorable to get a paper rejection?
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.