I agree with Cannon 100%. Not only is it going to look dodgy, after a short time wearing it the edge of the leather will lift up. This has happened with every pair of Nomex and Kevlar gloves I've had, even the best from Blackhawk (which run upwards of $60 retail). On a pair of gloves worn in a HMMWV or a Chinook, it doesn't matter, but on a watch strap it will. It's got nothing to do with the type of thread, the material or the leather. In fact a tougher thread is only going to wear through the leather faster. One call 'roll stitch' the edge to try to keep it from lifting up but my understanding is that this is particularly difficult to do with Kevlar and you weaken both pieces of material even more, resulting in more failures.
Nor do I care much about a Kevlar strap. Nylon has always worked fine for me (though some weaves do elongate a bit when wet as deepcdvr points out). If Kevlar is that hard to work with, it will be expensive and not given to user mods (length, extra holes, etc.). I'm more interested in the form of the strap (as shown above in the IWC and B&R photos) and if we can work the brand name into the weave, even better.