I think, at least here for XenForo, it's a mix of challenges that come with producing an add-on. I don't want to speak for other add-on authors here, so please read this more of how my experience has been here.
When I was creating the blog I obviously had to decide whether or not to charge for it. I was inexperienced with XenForo, and generally speaking I had never managed a proper add-on of any kind (even on vBulletin), nor had I any real experience with PHP besides "minor" scripts I made and very light extension of vBulletin's framework. Factoring this in, I decided not to charge anything as the blog was nothing more than an initial experiment. As I found my footing things got better and now Beta 3 is a very strong release over its predecessors, with Beta 4 planned to take that even further.
Now, would I charge for the blog at this point? Yes, but there's a problem.
XenForo encourages us to release our add-ons here, and with the future resource manager add-on that will be deployed here shortly, it will definitely help us organize things and support the add-on for the end-user and as a developer. However, there is a slight problem with it already and it's that there's no payment support for it. There may be such a feature in the future, but from what Kier [or Mike?] said it won't be there initially.
I could still charge for the blog, but then I'm stuck with managing payments on my own, handing out copies to customers (no longer just "users"), and keeping track of all that information manually. At this point I might as well just host the blog on my own domain somewhere and manage it through subscriptions or something, but then I lose the benefit of being hosted right here on XenForo's company site and losing exposure by being located elsewhere, not to mention it would require people to register in another place.
So what did I end up with? Leaving a donation link for people to support the add-on if they'd like to. It was really the only way I could think of that would simplify things for me, by allowing a small amount of income from my work but not requiring me to juggle far more than I'd like to and allow me to focus more on developing the add-on itself.
I guess in the end that's the point of what is being talked about here. Would you pay if you had the choice to or would you pay if you were forced to?
Do not take this as some sort of cry or plea, but so far I will say that it seems a lot more will probably buy the add-on if they had to without complaint than simply download it for free with the option to pay whatever amount they like (or not at all).
I do understand the worry about donation-based/free add-ons, but at the same time it's that fear that also ends up being the death of the project because the author has to abandon it for something that will actually pay the bills because people are afraid to support the author for fear they may flee. And, if you're thinking making them paid-only prevents such a thing, I've seen the same thing happen there too.
I don't want to instill fear or anything, but yes there's a risk involved but it's on both sides, not just one. Making a free/donation add-on is just as risky as making a paid one, though there are differences, and for users it's always going to be an unsure thing one way or another. I guess with money on the table they feel more secure, but that doesn't mean it's nonexistent.
Look at it another way: XenForo came out with two lawsuits against it by a much larger corporation, and yet there are a bunch of us still willing to put out money to buy more licenses, unsure 100% that XenForo will survive. I'm not saying blindly throw money at something, but seriously consider the add-on or product you're looking at before you immediately write it off as something you'd not risk just because it's free or requires a payment.
That said, I think it's time I get back to working on things…