I'll probably come across as a heretic to all the 16-35 owners... but I have the 17-40 f/4 L and I highly recommend it. I don't think a superwide really suits portraits that well; longer focal lengths seem to do that better, particularly between 50 and 100mm. You seem to have those lengths covered pretty well. The superwide is really better geared towards landscapes, in which case you're almost always stopped down to f/8 or lower so you'd rarely if ever need the 2.8 for landscape shooting. Just my two cents.
all valid points
that is why i prefer to use the 16-35 II on APS-H giving 21-49mm which is a really nice range for events and as a walk around, I prefer this approach to using 24-70 since it gives me near enough to 50mm on the long end and 21mm is Just that bit better than 24 on the wide end, also croping the corners means its sharp all over at f2.8 for events etc, colour and contrast is also stunning on this lens. I originally got it for landscapes but soon discovered it can do sooo much more than just that.
another massive benefit is its all internally zooming unlike the 24-70