If Xenforo offered direct integration with Authorize.net or other merchant account providers, then that would become a
big differentiator and selling point for Xenforo. So far
no other forum software uses anything except for Paypal, and many forum owners are looking for another solution. There have been horror stories of paypal holding accounts hostage and cutting off streams of revenue.
Also, many people are
already using the forum more like a full CMS with the portal add-on, etc., and want to set-up a paid membership site and control access to their content and handle their own payments. Thinking of Xenforo as simply a forum software is very limited and short-sighted. There is no reason why it can't be much more, as all of the basic components are the same (text, video, community interaction, content, threads, profiles, etc.)
Look at Membergate at
http://www.membergate.com (a membership site platform, which costs hundreds a month). With a few tweaks
Xenforo could easily replicate their full functionality at a fraction of the cost and capture the paid-content market and people looking to start their own membership site.
Simply adding
Authorize.net integration would be a good start, other things could come later (CMS, etc.).