Wikipedia on Rewrite (programming) has some interesting references where people discuss the pro and con of rewriting a software. It's worth reading those, if you don't have experience with rewriting a project of this scale.
From my own experience, I can say that a rewrite is a good thing if it's your software, if it's messy, if it's small, and if you know exactly what you can do better than in the first incarnation.
For big projects, rewriting is a pain. The project has so many files and features, you don't know or don't remember all of them. Even worse, there may be tons of third party applications that depend on your project in ways you don't know. Rewriting such a project means throwing away (and having to redo) years of work, experience, testing, and bugfixing.
I've seen both cases... I've succeeded in rewriting things, and the result was much better, more performant, and easier to extend than the old ones... but I've also failed in rewriting things, usually huge projects, simply because it's too much work, takes way too long, and if you manage to finish it after years you're back to status quo - while the new code may be much nicer than the old one, essentially what the software does is still the same, because you concentrated on rewriting it, and not on adding a whole lot of new features.
In case of MyBB, I don't see why a complete rewrite would be necessary. Compared to many other projects, the MyBB code is fine. I've only started using it with version 1.4.3 or so (shortly before the 1.4.4 release) and during this time I wrote two plugins. MyBB code is easy to understand, easy to modify, and easy to write plugins for. And the software itself works well, most of the time anyway.
Rewriting this from scratch seems like an awful lot of work with very little benefit to me. The time may be better spent picking one component of the MyBB puzzle at a time and giving it not a rewrite, but an overhaul.
MyBB has other issues that should be worked on instead, for example lousy project management. Bug reports are getting ignored, security issues are brushed under the carpet.
