One way spammers find a place to add backlinks for their site or your site which you ask them to promote is to look for “Powered by phpBB” on major Search Engines. The phrase will be on sites using an Internet forum package and since removing the line from the online community could have support not provided, many choose to keep it. Others leave as they do not know how to remove powered by details.
We request you retain the full copyright notice below including the link to http://www.phpbb.com.
This not only gives respect to the large amount of time given freely by the developers
but also helps build interest, traffic and use of phpBB3. If you (honestly) cannot retain
the full copyright we ask you at least leave in place the “Powered by phpBB” line, with
“phpBB” linked to http://www.phpbb.com. If you refuse to include even this then support on our
forums may be affected.
With the amount of exploiters and spambots out there, I would get rid of it before the website goes public or offered on an established site. Also, you could temporarily add it in for support then zap it away once your issue has been resolved. If you are creating the accounts for clients and restrict members from registering themselves, leave it in if you have ads displaying as you can pick up some page impression from the visits.