I’m all for QRP operation. The rules says something about using no more power than is necessary, and that’s a good idea for a number of reasons. It’s probably a good idea to limit your RF exposure as much as possible and the lower power that you run, the less chance you’re neighbors are going to hear strange sounds emanating from their stereos and computer speakers. And, of course, when operating portable, running low power will help preserve your batteries.
BUT…
If a station is not mobile, once a QSO is established, the QRP station should QRO if conditions are marginal. It certainly would make it easier for the QRO station operator. I have been in many QSOs where the QRP station could hear me just fine, but it was a bit of a struggle for me. The courteous thing to do would be to switch on an amp at that point.