First off, if you are installing mid-way between two releases you basically have 3 months before your distro is obsolete. I just put 10.04 on my laptop in July and already I'm thinking about upgrading again (which has not always been smooth, 9.04->9.10 sucked), and I haven't even gotten around to my desktop yet, and now I just decided to wait until 10.10 because why bother doing it now? So I'll still be on 9.10. It becomes a hassle for users after a while. I remember when I was downloading the release on the night it came out and updating my machine, but the releases these days are less and less noticeably different.
I also wonder if they could accomplish more in 9 months than in 6, some percentage of time is spent doing overhead for each release and a slower release schedule would allow them to bite off bigger chunks. 6 months made a lot of sense when it was a new project and there were a lot of low hanging fruit, but these days I feel like the path forward requires bigger steps and more time.