I think there are two distinct camps of bike-shopping triathletes: 1) those for whom this will be their only bike, or the bike that they do the vast majority of their riding on, and 2) those who do most/all of their outdoor riding (at least in the offseason) on a road bike.

I'm in the latter camp. I have a road bike with DA Di2 and disc brakes that is a dream to ride, and perfect for all the hilly riding around here. I love riding that bike outdoors. My tri bike is a Cervelo P4. That bike is an absolute missile and I can't imagine there are very many current bikes that are faster than that bike, even though it's DA 10sp and rim brakes. That said, the P4 flat-out sucks for the big climbs around here, and sucks even worse on the descents. I would never dream of having the P4 as an only bike with all the hills around here.

So I guess you can put me in the camp of the not super-interested in a tri bike with disc brakes (though I wouldn't *not* buy one because of it) but I sure as hell wouldn't ride a road bike without them again. If I was looking to consolidate down to a single bike for training and racing then yes, I would want disc brakes on my tri bike.