When the 787 comes out and airlines will renew their fleet, they will get rid of their older a/c's, won't they?
The renewal fleet isn't for tomorrow, if you read well LindbergB767, it's 2010, not now.
Plus, it's more expensive to fly with older a/c, the 310's are or will be 20 yrs old, so it costs a lot of $ to keep running those, while you have modern a/c's with less fuel consumption.
Of course one 310 is for this winter and another one for the summer, like Gumbi said.
I guess we will have to wait and see what's the future will be.