There is nothing, absolutely nothing, you can do on a i5 and can't on a C2D.
It's only a matter of performance, and the difference is measured in seconds and, at worst, minutes.
That means that yes, in 3-4 years a C2D will be obsolete, but the same is for an i5.
Don't trust on the "all-about-specs" guys that live in forum like this: they are people witha real passion for computers, and they make you think that if you don't have the latest top-notch hardware, you are doomed
this is not exactly true in the real life
BTW I can understand that in 2010 a C2D it's not so attractive
it's value for your dollar
right now i can get a fully tricked out system from Dell or HP for the price of a MBP. 1GB graphics RAM, a much faster graphics chip, same LED screen, etc.
on the low end iMac, same thing. the i5 iMac is OK only because they monitor is like $1000 if you buy it from Dell.
Apple isn't paying top dollar for these parts because they are old and almost end of life. why should i pay a premium for them when others are selling modern technology?