Having seen the recent line of Canon's MkII L lenses, I'd expect a considerable improvement in IQ. That alone is a big factor for many to fork out the extra cash. Didn't we have similar discussions when 24L MkII and 70-200L IS MkII hit the market?
The other factor is repairability. No matter how good MkI is, in due time when its spare parts become rare, using it for professional jobs becomes risky. That's when pros need to move up at the latest. We are still a long way away from that time though.
I bought my MkI about a year ago and am very satisfied with it. If I don't lose it through accidents etc, I wouldn't be buying MkII in a while. That said, I thought the same about my 70-200L f/2.8 (non-IS) and then IS MkII made my jaw drop - some time later, I gave in.