I don't see the logic or morality of denying credit and copyright to the developers of software. I wouldn't remove the author of a book so my visitors couldn't see it.
I can understand it if the text line is large, brightly coloured, or in the header. But it's not.
Nobody is taking away the copyright by removing the branding line at the bottom of the pages. It's branding, not copyright. And for some (in my experience not so few) companies script branding is not something they like or want.
To me it's also a design question. I like to have as few lines as possible in the forum footer. But I usually don't go so far as to get the branding-free option. The price usually isn't worth it and I just convert the text to images. I only bought the branding-free option for one of my vBulletin forums.
Some people also don't like the fact that people can get to their forum by running a search like "powered by Xyz Inc". That is how many script kiddies and spammers look for forums to attack with their crap.
It's not security by obscurity. If there are security holes, an attacker can abuse them. But a lot of the time script kiddies just do that for the heck of it. They don't want to attack YOUR forum. You just fall into the net that they cast.
Not displaying branding information is one way to stay out of that net. Not always, but a lot of the time. I'm sure there are other ways, like looking for filenames exclusive to a certain forum script.
Still, it's enough for me to convert text-branding to images.
When I check out software I look for the dev name if I want to investigate it further towards buying it. That's a HUGE chunk of value to the developers.
Correct, which is why it's ok to charge for branding-free.