I have submitted a paper at a certain conference that has a rebuttal phase (authors are presented with initial reviews and can write a rebuttal, which will be used for the final decision). Based on the reviews, it seems that the chances for the paper being accepted are zero. The submission system of the conference has a "Withdraw" button, which makes it easy to withdraw the paper and submit it to another conference. I intend to make use of that option, after using the available reviews to significantly improve the paper.
To my surprise, a senior colleague explained to me that doing so might be considered unethical. The explanation was one could see it as a wasteful use of reviewer time, comparable to a double submission.
This explanation did not really get clear to me: Assuming that I use the reviewer comments to improve the paper, I have used the reviewer time in the intended way. The situation would be exactly the same as if the paper had been rejected right away, and I had proceeded to submit to another conference.
Am I missing something? What are the ethical implications here?