I have the potential opportunity to buy a full carbon cross bike (fairly new Specialized Crux Pro, specs say around 17 pounds; not sure what it would be with pedals, etc.). I currently own (a mtn bike which is not part of this equation) a cruddy commuter, a Kona Jake 2010 (24 pounds!), and a Giant TCR2 2005 (21 pounds with aeros on). I race all of the bikes (ok, not the commuter), but my primary racing on the road bike is in triathlons where I throw on clip-on aerobars. My Jake weighs a ton, and I'd like to switch that over to being my commuter. But I also would like something a little lighter for my triathlons, and the compact geometry of the TCR2 doesn't work well with the aerobars. Would buying the carbon Specialized get me closer to my goals in one fell swoop (budget is very limited, but I'd get a second set of light road rims), or just get a new road or cross bike? I have some dream of someday buying a tri-bike, but that's a long way off. I'll also add that one of the things I really don't like about the Jake is that the brakes suck, and it can be downright scary on some trails. If it helps, I'm a female.
N + 1 is always the answer.
Seriously though, if you're going to actually use it for cyclocross then I'd advocate buying it as this would be a substantial upgrade. Lugging 5+ pounds less of a bike over obstacles, up hills, etc, etc over the course of an hour means a ridiculous savings in energy.
Will this upgrade your cyclocross bike (Kona Jake)? Yes.
Will this upgrade your road bike? Doubtful. The weight difference won't matter as much as the wheels/tires/cockpit/general configuration.
Will it double as a tri-bike? Doubtful. Tri-bikes are just too specialized.