I should elaborate:
I'm running an almost 3 year old MacBook Pro. I do all of my development within VirtualBox (Ubuntu 10.11). I have it connected to a gasp single external monitor. I work every day on a MongoDB/Python stack.
It could be faster, sure... but I'm hardly unhappy with it.
I think that for most of us, a few gig of RAM, 2 cores and a 22" monitor (that was my setup last month) is enough that more horsepower wouldn't make us substantially more productive.
jasonkester mentioned "crap hardware" versus "good hardware", a pretty minor revision of a machine likely won't move it from one to the other.
Clearly you get diminishing returns here. And in your example you selected the comparison points (MBP'10 vs '11) so high and close to each other that the effect is negligible. jasonkester on the other hand is comparing "crap hardware" and "good hardware"...there the effect is huge.
// haven't tried putting the VM's on the Promise RAID yet