I believe the point of Plan 9 is not be a standalone workstation or server like Unix or Windows, but be a fully functional network client OS for network services...
Kinda like novell's nds client for windows/*nix, but instead of being a tack-on to a more complex OS its the main point of it's existance.
I am not sure but the concept goes like this:
Unix is designed as a multiuser enviroment. You have a central Unix Mainframe/server, on which a multitude of terminals and X clients and stuff is added on for the Users. Everything is centralized, files/applications/cpu power etc.
Windows was designed as a single-user system were you own your computer, or you operate the same computer day in and day out. It's "your" PC. NT was there to provide a way to share information between PC's.
Plan 9 was designed for a completely networkable multiuser system. Unlike Unix, the servers are distributed and everybody gets there own computer.... I guess trying to get the best of both worlds.
but all these OS's have morphed and twisted their way to fit into the changes of technology and sociaty's needs, so I suppose the number 1 reason for plan 9 nowadays is that it is a freindly (note the rabbit with the helmet) and usefull, completely networkable, productivty driven OS that is not from Microsoft, but may be nicer to use then Unix.