Saying "This is a legitimate bug, but I don't care enough to fix this for you." is already an order of magnitude more polite than "Pull request welcome" or the older "Patch welcome" , explaining in details why and if necessary how open source work even more so. You have to remember than not every one know the Open Source community speak. If you can guide the reporter on how to create said pull request, even better.
Yes its take more works and it's less fun than hacking at code, but building a great community is a lot of works. It's also, for me at least, what separate good projects from great ones