Clearly there's benefit to wearing a watch and even a heart-rate monitor for interval training, but other than that, why clutter what should be mind-clutter free time with more technology like a FitBit?
Some people still prefer an actual wristwatch to digging in a pocket to activate a cell phone screen. And if you're going to wear a watch anyway, why not wear one that also motivates you to get more exercise and has caller ID built in?
If your phone is truly your only device, then you can make it approximate the functions of lots of other things (though it's not as good of a pedometer as a dedicated pedometer). But I think there are probably some useful UI advantages to having a more easily accessible screen on your wrist for things like the time, silent vibrating alarm, calendar alerts, heart rate monitor, and caller ID. The key is going to be making that second device sufficiently stylish, like a watch is normally considered jewelry, and sufficiently cheap that it can be replaced every five or six years as the technology improves. I'm not sure we're there yet, as the good devices are expensive and the cheap devices don't have all of the functionality (like the fitbits don't do calendar integration or GPS).
At the very least, I'd much rather see a 2025 where everyone gets turn by turn navigation read out to them by their wristwatch than one in which everyone is still stumbling down sidewalks with their noses glued to their cell phones. I think there will always be some functions for which a phone/tablet is clearly superior (all media consumption, for example) but for others I think the watch form factor will actually be superior.
Predicting the future of technology is hard, but it wouldn't surprise me if we start seeing phones and watches sold as matched pairs with complementary functionality. Samsung and Apple are both already leaning towards complementary devices, but they're not yet selling them as a matched set. The current divide between smartwatches and fitness wearables is dumb, and I think it will rapidly disintegrate as they become more and more similar. This doesn't bode well for fitbit as a company because they don't make phones too.