I've got interrupt brakes on my bike. I've had this bike for about 3 months, and I think I can count on one hand the number of times I've used the interrupts. I do occasionally top-bar, but it doesn't ever seem to be in places where brakes will be needed, or maybe I just always instinctively go back to the STIs when I need to brake while top-barring. Not sure.
I suspect they probably add some weight... not a pound, but also not an ounce. I feel like I'd just as soon have them off, have the benefit of more top bar room, and less overall weight in the spirit of rolling as light as possible all the time. But it'll be a pain in the ass... probably have to replace the cables and definitely have to replace the housings, as well as rewrap my bar and so on. And maybe if I trained myself to use them, I might actually appreciate them- I pretty much never ride with my hands on the hoods, always either dropped or on the top, as I have my STIs mounted really low for better reach while dropped.