I don't know how you can manage 1/2 sec. shutter speeds even with IS.
I'm coming from a different perspective: About 60% of my jobs don't allow me to take pictures during the ceremony because the resorts have their own photographer who has sole permission to shoot them. Therefore, in most cases I only shoot before and/or after the ceremony, doing posed shots and motion shots.
For the reception, any shutter speed will do most of the time because I'm using flash, but the lens needs to gather enough light from my flashes and I do set them around 1/4 power most of the time to conserve power and decrease recycle time. Sometimes I set one or two at 1/2 power. This usually gives me around f/2.8 ISO 800 for a properly exposed shot. I really don't like to push to ISO1600 if I don't have to, but I know the D700 or 1DIII would be able to handle ISO1600 all day long.
The other environments I shoot in are casino resorts and I can't set up flashes, stands or other lighting equipment in those places because it's both a foot-traffic hazard and against the general policy of the resorts to allow paid photographers to take pictures on their property. I also don't have an assistant to make that kind of thing easier, in part because I don't charge enough to pay an assistant and still make money. Therefore, I am left with the option of getting the fastest lenses for the job to get as much light as possible. I'm probably going to end up swapping out standard zoom entirely for 35L and 85L.
Then, just last night there was a lighting situation by this grid-patterned wall of light on the strip, and it gave enough light for f/1.2 at ISO800, around 1/125 sec for a really good exposure. f/4 would've been too dark, I would've had to push the ISO up to 1600 to get 1/25 sec. f/2.8 would've given me around 1/50 sec for the same exposure.
I think that if they make a 24-70L with IS, then the standard zoom might once again have some definite usefulness over the primes for me in posed shots where the people aren't moving, but so much of my work is at night or indoors with available light, and primes seem to be far more versatile for that.
Now, I'm less experienced, so this all may just be part of a phase...but right now it seems like a good idea for me. In the future, maybe I'll think something else is a good idea instead, and I'll come around full circle.