I shall use my perogative as owner of this site to have the last word.
Do you, or do you not want developers to release free add-ons? If you are happy to pay, then maybe you can negotiate terms of how their credit or copyright is displayed, but I doubt it. If you are getting the code for free, then you damned well better display their credit, out of gratitude if nothing else. And if that means a link in your footer, then that's absolutely legitimate. Want to get rid of that link? Negotiate a respectful sum with the developer, or use a different add-on. No alternative add-on available? Then you're going to display that link in your footer, or your site is no better than a pirate.
Do I have objections to a 'powered by' page? No, not at all, it's a great idea. Is this an excuse to talk about how terrible it is that some developers want their credits displayed in your site footer? Absolutely not, and I am horrified by some of the attitudes described in this thread. I'd prefer not to have my code running on your site. It is perfectly valid for a developer to demand a credit in your site footer for software that you run that helps to run your site. Don't like it? Fine. Go elsewhere, but you have no right to complain.