I am a little offended by James' implying that people who buy branding-free are somehow trying to sabotage XF's sales. Nothing could be further from the truth. Buying branding-free is about your own brand, not about XF.
When I design/code a client's website (or install a forum software), it doesn't matter the least bit what I am proud of using or whatever.
If the customer is fine with the branding in the footer, they're fine with it. If not, they have to pay for branding-free. However, I have never-ever had a client, that didn't ask me to remove the "powered by Wordpress" line from the footer. I too use and love Wordpress. But the license under which Wordpress is distributed allows for de-branding.
Putting Wordpress in the same pot as XF doesn't make much sense, because XF is commercial software and both the reasons and effects for/of having branding in the footer are very much different.
For personal projects it's a pride/fanboy thing. But for clients it is never that (in my experience). They tend to not want branding on the site that isn't their own. Especially if it's free software.
And that is not as terrible as it sounds, because:
A) anyone that knows their way around Wordpress will always be able to tell if it's Wordpress or not. And
B) people who don't know how to do it will either learn (and then become A) or pay someone to do it for them, who is very likely to be A.
What I tend to do with most of my personal projects is I put the branding in the (in Germany legally required) imprint/about page. That is where people will look for that information anyway. It's not about de-branding though. I just don't like branding lines in the footer and I move them to the about-page whenever the license permits.
However (to make the way back to the point) that dislike is not so strong that I would pay money to remove (instead of move) that branding. So (apart from one forum) I always leave the branding in and don't buy branding-free. I probably won't buy branding-free for my XF licenses and I doubt that any of my clients will either. Especially since the vast majority of my clients don't want or need a forum.