So you're not actually doing everything (or even most things) that Dropbox does. I mean we also have a file server with a several TB of disk and gigabit links and VPN and all that jazz at the office, but that is in no way a replacement for what Dropbox offers.
For us it is important to work on the same files. To make them available to our co-workers, to have the same versions, etc.
Syncing is a mechanism. If it would help us to achieve our goals, we could use it. If some other mechanism achieves our goals more efficiently, we would use it instead. Syncing in itself does not have value to us.
Dropbox and a NAS or file server fulfill different needs with not that much overlap. Sure you can probably hack your NAS to be a bit like Dropbox and perhaps you can hack Dropbox to work a bit like a NAS, but a the end of the day they're complements not competitors. If you don't need what Dropbox offers that's cool, but that's not the same as saying that Dropbox doesn't have anything to offer over a NAS. Personally I use both and would never want to trade one for the other.
The data is available over gigabit link. It is local, after all. No need to sync to hide the latency.
And after all, there is more data on NAS, that the drive on my notebook can handle. No need to have it all locally.