For what it's worth, as others said, when shooting in a "flash/strobe" environment, the flash will stop the action and essentially IS is a non factor in the results of the picture. IS does help framing the subject up in the VF though, which may be a benefit if handholding in a studio. At 2.8, I believe the Canon's 24-70 II trumps all competitors in this range. For $2100 it better trump all. Most portraits, in my experience, from 5.6-8, you may be hard pressed to tell the difference between any of the lenses... Now whether you shoot all studio, or if you shoot on location, day to day (whatever your hobbies are), non-flash environments like wedding ceremonies or museums or the like, then IS may or may not help ya... IS may buy you 3-4 stops in ideal environments, where the 2.8 only has 1 stop advantage of the F4, BUT, IS wont stop action whereas, its really debatable if F4 doesn't stop the action, would 1 stop REALLY stop the action or not?
So... the 2 factors you have to weigh is the 24-105 and tamron is nearly similar prices whereas the 24-70 is a cool grand more expensive than either of those two lenses... and if you find 2.8 the holy grail, would you be able to live with tamrons inferior body and build and 2.8 sharpness for a $1000 less overhead.
As for me, both Sue Bryce and Sandy Puc endorses the 24-105 for portraiture, and give their sales averages, they are doing something right. For my everyday work, it serves me reliably, but you have to make your own choice.