Yeah it's nice, I'm just at a loss as to why it's not already incorporated into Google Fit. Just doesn't make sense.
There is probably some data that shouldn't be merged, the main one I can think of is steps. Google Fit is using the pedometer in your phone, yet FitBit is using the one built into your band. Won't talk for everyone, but I wear my band more than I carry my phone. Which steps should be used, which steps are duplicated if you're carrying both? You can imagine the issues that could bring up.
Google Fit could integrate it's auto-calculated walks as workouts into FitBit, I'd love to see that, but both-way sync could be problematic as they could be duplicated.
I have issues with that now, MyFitnessPal syncs to my Google Fit, Google Fit is putting my walks into MFP, but MFP syncs to FitBit so I'm getting duplicates workouts. It's at least smart enough and only one set of them actually has any calories taken out of my days allotment (I'm not even sure in which app that is taking that into account) which is a bonus, but still, I didn't walk twice the distance which it says I do.
The ability to not use one and only use the other, now that would be something. Dropping Google Fit's reading of the phones pedometer may be possible, not sure on if you'd want that or not though. Having to tell it when you've stopped using the FitBit (or it's battery is dead), or you moved to another device could cause data to be lost.