Addons and themes that cost, my opinion harm the attractiveness of the Forum Software.
Sorry, I split it because it can become a lengthy discussion a little OT for the original topic. ;)
I'm not an Open Source Taliban. I like release software that can can be used without restrictions, but it's not that if someone asks me to create something for bucks I say "no". To me it's fine to gain money through coding.
That said, looking around, it seems to me that the most widespread software (e.g. wordpress, joomla, drupal) have an ecosystem composed of both free and paid resources, and none of them is really affected by the fact paid-for "extensions" and themes are available.
Why have paid addons and themes would harm the attractiveness?
I think, but that's of course just my opinion, that it's the other way around: paid resources appear when a project is popular enough to generate revenues (otherwise, someone looking to make few bucks would go somewhere else). And, the moment a project is popular enough to have paid-for resources, it means it is likely used also by people that consider putting some money into what they are doing.
I personally think that exchange money for services is absolutely normal and should not be a stigmatized and considered Something BadTM.
Of course it's just my personal opinion. ;D
My opinion: Unfortunately a widespread bad habit, everything want to have as a gift.
It can be easier to have the opinion that it is "bad" when you are the one offering it for free - but I am with emanuele and netflag: theres nothing wrong with paid themes/addons generally, its inevitable once something become popular enough..
My only beef is with paid addons/themes that clearly are of inferior quality. :P :) Some people like to make a quick buck, but not put the time into it. Or even posess the skills to justiy a price tag...(..). But, my opinion only of course.
The costs are more likely due to time spent to research and write such a good code. So no harm to me either. Most people can think and decide whether to buy it or otherwise use a free service or write their own code.
On joomla I have a few paid components that I install in most of websites and I see nothing wrong with it. I understand that there is a lot of effort and time put into a complex addon so for me no problem to spend some money and support the developer
If you look at xenforo most of addons are paid, no problem to pay for a gallery but I'm not confortable to spend 5-10$ for a "users online today" addon
Well, that's "supply and demand" at work. :P
Small-ish addons are usually used as demo and it should be unlikely they are paid. And anyway someone else may make them for free. :P
That's why: Eyes on when addon purchase. ;)