i don't use a flash all that much - maybe not enough because it's a nuisance to carry a large flash. and except for fill i don't like the results of on-camera flash
the 7D has the very useful ability to trigger a remote flash with it's pop-up, although flash balance is up to the external settings of the off-camera flashes - if they have that capability (i often use a diffuser on the pop-up) - as far as i can understand - and the same is true of the 90EX, except that the 90EX forces a choice between on-camera or off-camera trigger, not both as the 7D pop up
the 90EX is not a whole lot larger than a pop-up even though it only has a GN of 9 compared to 12 of the 7D pop-up. forutnately the high ISO capability of the 5D3 pretty well makes up for the difference.
a not cheap addition to the already high-priced 5D3 (which i strongly feel should have had remote flash trigger if not pop-up). but it gives me a pocketable fill option or trigger for the (also pocketable) EX270ii or other off-camera flash(es)