Why is a second edition lens long overdue? I remember there use to be a time when people would buy a lens and never dream on needing an "updated" version. Don't you think this might just be consumerism and something that really is not needed. The only reason why I am on the bandwagon on waiting for an "updated" camera is because current dSLRs still lack the quality of film. They are getting close and on that day it is comparable, I think I will just have to run that camera into the ground.
I guess my point of what I am saying is I have known quite a few successful photographers who just shoot with one lens or just one old S___ty camera and still produce great results or have a incredible style. I guess I am just having a tough time understanding the importance an extra little gidget or gadget will make over the long run ;o)
I think consumerism plays a part here, but ...
Note Canon has very few lenses which are mk3, from the top of my head only the 75-300mm which was last updated in film days.
My impression is that the benefit from today's computer power drives much of the upgrade to mk2 lenses:
* Electronics have progressed, allowing for faster USM, improving IS, etc. This is no different than the upgrades Canon made in the transition from FD to EF, such as autofocus and allowing the camera to control the aperture.
* Ray tracing allows to create improved optics.
* Digital makes it easier to fix things which couldn't be fixed in film, making a different set of trade-offs in lens design more attractive. As example, it's easy to fix distortions (barrel, pincushion, etc, all up to a point) with a computer, so making a lens with less chromatic aberation and more distortions makes sense.
The ability to improve lenses raises expectations, and this is natural, just as in other areas of life (need examples ? I'll be happy to produce a few).