Why have phpBB3 + Joomla! When you can have phpBB3 by itself!?
phpBB3 has a very customizable "engine" that allows it to be used as a very powerful CMS... You can easily add a portal such as Stargate Portal, or something that will make your forum fully integrated with your portal...
I'm not writing this just to advertise phpBB... But it is good software, and with just a few lines of code, you can almost integrate any website into phpBB3...
So... Even though Joomla! is pretty powerful, it will still need a bridge so that it can share sessions, etc with phpBB... I have tried many bridges and the only bridge that is quite stable doesn't work very well... And if people have profiles, they would still need to update both sides...
But... If you have the time to mod phpBB, then by all means do it, but if you have no option (or time), go with Joomla! and phpBB... A solution to not use the bridges would simply be to disable registration on the Joomla! board...