To understand why bbPress will never be a plugin, you have to understand bbPress history.

bbPress was written out of parts of WordPress code by Matt (in just a few days) to replace minibb on WordPress.org. It was released a couple years later as 0.7 under GPL so it could benefit from the free labor of community contributions, just like WordPress.

bbPress 0.8 and 0.9 actually corrected several of the "mistakes" in WordPress that have become a burden of legacy and backwards compatibility. But with 0.7-0.8 integration was not a main focus and no attempt was made to make it particularly easy, though it was possible. By 0.9, integration had become quite a bit easier (that is until WP 2.6 then 2.7 then 2.8 kept changing the @$%!@$ cookies and breaking compatibility).

Matt then decided to have TalkPress created, which is like a forums version of WordPress VIP - Automattic hosted forums for premium customers. However he wanted an easier to maintain code base between the projects and object based caching like WordPress, so he directed that a subset of WordPress code be created (backPress) and replace the existing code in bbPress.

So bbPress 1.0 as it stands now is a rather different creature internally than 0.9 - it maintains most of the 0.9 compatibility thanks to a massive effort by Sam (but still breaks enough to be a pain in the @#$ to plugin developers). However because of the backPress bloat from WordPress, it's now 50% bigger than the relatively lightweight 0.9 and takes 50% longer to generate most pages due to it's increased complexity to interface with backPress.

Until WordPress itself is changed to also use backPress therefore sharing code during integration, and to the best of my knowledge there is no such intent stated on even the far future timeline, deep integration of bbPress into WordPress requires a massive amount of code to be loaded and executed for every page (nearly 1 MB per instance) and at least 30 queries per page BEFORE plugins do their thing. Such a load would get a user kicked off most shared hosts without caching on a busy site and forums don't lend themselves to page caching as well as a blog because members typically get non-cached pages.

So no, bbPress will never be a plugin for WordPress.
(but there will be a plugin for WordPress to automatically INSTALL bbPress, which I think is a different animal entirely than what you are asking)

ps. all of this is simply my opinion based on observation and experience