Talk about bad luck... man that sucks... I got my 2nd Xbox 360 in December 08 (the 1st didn't rrod, I just sold it). The manufacturing date on it is September 2008 so I'm hoping it has the 'Jasper' Motherboard in it.. They are supposed to finally put the rrod to rest.... so here's hoping.. I could just take it apart and see but I don't really want to due to extreme laziness.. hahaha
As for how I use each OS I have installed:
I mainly use Windows XP for old school gaming... I know about compatibility mode and all that but some older games just don't work well unless their on XP.. period.
I use Windows Vista mainly for trial and error with new apps... Since XP and Win 7 are both very stable OS's and very reliable I'd rather use Vista as the guinea pig for new apps to be tested on.. (new apps as in apps that I've never used before)..
And Windows 7 is quickly becoming my everyday/everyuse OS.. Once it's in final form it will definitely be my main OS of choice.. but like I've said before, I'll still use XP and Vista somewhat..
I also use Ubuntu alot, for multiple things.. If you've never used linux then I'd suggest downloading Ubuntu or a variation of it like Kubuntu or Xubuntu, you won't be disappointed...
Your right in saying that dual-boot CAN be a waste of time.. that is if someone was to install XP and Vista (for example) but then only used XP all the time and Vista just sat there.. then yes that would be a waste of time and resources (HDD space).. but if you have a valid use for each OS that you install then I don't think it's a waste of time at all...