It's really based on the environment. Generally speaking an on premise distribution point is not open internet. Depending on the nature of your footprint, an on premise distribution point may not function at all.
We killed our on prem DP this past summer, and have not looked back. We still keep a package repo just in case, but its not deployed from.
@jsommers There's a huge amount of "it depends" in the answer to your question. First off if you do keep any on-prem DPs you _really_ should enable HTTPS on them as if will provide much more performant downloads than SMB (no mounting of the file system, and if there is a hiccup during download HTTPS will resume a download).
If you're not dealing with re-imaging labs, or anything else that involves deploying large installs (e.g. Adobe, Office, other things with multi-gig installers) to multiple machines simultaneously then on-prem DPs probably aren't especially necessary. It also helps if you have a large pipe to the outside world from your campuses.