Although this might seem like a forum centric site, in fact it is far more than that. Although i have no insight in the actual number of roles on this site, it must be a huge amount by now: a few for different team roles: corporation, trustee, core team, project team lead, project team members (one per project), core team alumni, different levels of sponsors, etc etc. These represent a lot of people. A small group compared to the 500000+ members, but still. So my best guess.. the many must suffer for the few... 
... just like it seems to be in real life most of the time...
When it comes to sites like yahoo, google etc.. It would seem that those sites have a very homogenous group of users that do not require a lot of security apparently. Even google asks for credentials pretty often, depending on the things you are doing...
So.. can we substract a feature request from this? IMO Yes... it would be very cool if we could set cookie length per role, instead of one length per application... If we could that, we could set cookie length for "registered users" to forever, and for site admins to "12 hours"..... Who will take up the glove to look into implementing this?