Given that the the two 70-300's are variable aperture and go to f/5.6 at the long end, a stop of the IS is given up anyhow at the longer focal lengths
Yes, both the 7-3 IS lenses are 300 f5.6 (and also at f5.6 @ 280). If you add a 1.4x to the 7-2f4, you have a 280 f5.6 as well.
and the best they provide is 3 stops (canon's claim) anyhow. I think it is fair to say, that the 3 stops is optimistic and not nominal performance. Actual performance would be 1-2 stops nominal. That means that they they only get 0-1 stops more than the 70-200 f/4
I haven't found that to ever be the case with any IS lens I've used. With the 7-3IS, I could consistently get 1/60th sharp shots (480mm FOV) and 4 stops and even 5 stops (1/15th) was not out of the question. IS is also very useful for certain types of panning captures. I know LeeJay (DPR) and myself have talked how we can get 5 stops (and sometimes 6 stops) and sharp shots with our 7-2 f2.8 IS lenses. The point being that Canon's 3-stop claim is by no means "optimistic", handholdability is highly user-dependent, and IS is very useful. Put the 7-2f4 at 280mm f5.6 (1.4xTC wide open) and either 7-3IS at the same setting with shutter speeds around 1/60th (3 stops on a crop body), and I guarantee you almost every person shooting this setup under the same conditions would get a whole lot more keepers with the IS lenses.
On the other hand, the IQ of the 70-200 f/4 is considerably better than a 'very good' 70-300. Yes, these are good lenses, but the 70-200 is arguably the best zoom that Canon makes
Agreed.
So, If this is the choice given, then it is a very easy one
I don't think it's that easy. I think someone could reasonably choose the 7-3IS lenses and have made the right choice for them. In closing, I generally agree with you that the L is a better lens than both; where I disagree is that the L is always the better fit for every person.