It also suffers greatly from the lack of engineers working on it - as in some parts have a lot of comments, some nothing what so ever. Some parts look very well done, some parts less so.
Back in the day, I saw the code (those leaks were real) and one zip was able to fully compile (and this is where the PowerPC version of post R5.03 came from, I used to have it compiling on my old BeBox - long gone now) and the other was a much later version, but the zip was corrupt and half the files were garbled.
DISCLAIMER: No I don't have the code anymore, not that I would never share it if I did, and no, I have never worked for or contributed to the Haiku Project.
Most likely it contains some parts of third-party code under non-ooen license (i.e. in device drivers, but not only) and maybe some subsystems were licensed in some form of exclusive form to others. Also they might want to double check that code not by "accident" looks like somebody else (i.e. the ext2 code might be inspired by Linux's) This all leads to review of old contracts and code while serving no business (besides marketing to a niche of nerds)
Access owns it. Even though they aren't doing anything with it, if they just released the code they would probably open themselves up to lawsuits over possible 3rd party code. So they would have to pay someone to go over the code to take out 3rd party stuff. That would take money for which there is no tangible benefit to them so they won't do it.
"The only BeOS code that has made it into Haiku are Tracker and the Deskbar (the file manager and the equivalent of the start menu/taskbar, respectively)."
No. They rewrote the whole thing (minus the deskbar/launcher). If it looks to you that they're just keeping the old system stable, that'd mean haiku has been very successful in recreating it as open source.
They're close to 1.0beta1, which is defined as feature-complete for 1.0. It's after they do get 1.0 that they plan on really taking the system's design forward.
Not that it doesn't already have a few fundamental improvements, such as the integrated kernel debugger, the support for newer architectures or the package manager.