I'm probably wrong here but because of the light gradient and the text being white, the need to give it more prominence was needed and was actually implemented to entice readability. For me personally, I have a dark enough gradient that doesn't warrant a text shadow, and the light text on a dark gradient within my custom style looks fine without it.
The main issue (rather than just say I don't like it) and give the reason why. The text feels like it's hovering over the gradient (if that makes at all sense) and thus doesn't feel like it's sitting slap bang on the gradient. That's just me, though I tend to be a bit anal about the little things that others would likely overlook.
Either way, I can understand why this was introduced, and I also feel it's one of those things you like or don't which conveys across that your not going to make everyone happy. I'm fine with it personally, not a biggie that destroys my browsing experience.
I can see the argument either way to be honest. I suspect that Mac browsers do a better job with the shadow than Windows versions too... so I'll need to check that out. It looks pretty dreadful in the screengrab in the first post to this thread.
Right now I'm 50-50 over whether to keep the change or not. I seem to like and dislike it with equal frequency when I view the navigation bar.