That is a two-edged sword. The data is also inaccessible outside of your company. There are ways to make it accessible outside the company (VPN, WebDAV over https), but they tend to be complex, fragile, and sometimes unworkable (see next).
> standard protocols (SMB, AFS, WebDAV, whatever)
Support for the standard file sharing protocols (SMB, NFS, I presume AFS, and WebDAV) sucks or doesn't exist on mobile devices.
Also, most NAS vendors provide mobile applications, so you can access the data. They realize, that the standard protocols on mobile devices are lacking.
Anyway, to pay someone to get you such a NAS and configure everything for you is still a fraction of cost, that you would pay for cloud providers.
> to pay someone to get you such a NAS and configure everything for you is still a fraction of cost, that you would pay for cloud providers.
Dropbox for Business costs what, $75/month for 5 users? That's less than you'd pay for an hour of a competent person's time.
I'm not a huge fan of Dropbox for several of the reasons that have already been mentioned above (I use SpiderOak myself), but on these specific points they definitely beat the roll-your-own approach.
If you are a services/consulting company and do client work on-site, you often have to sign an agreement from the IT department that prevents you from using a VPN on the client's network. In those cases, your remote workers need web access to files.
We have access either to completely separated guest network, where we have to use VPN to both our network and customer network, or access via Citrix or Remote Desktop, where to exchange the files we have to use the built-in file share facility.