Allow me to demonstrate under the latest macOS (10.12 / Sierra):
1) Go get the screensaver bundle.
2) Open the .dmg
3) Now, from the drawer with all the screen savers, drag out Pipes.saver to your desktop. It's perfectly safe. Double-click it to install it.
Here's what happens:
First, you get a dialog that says "can't install pipes screensaver" from preferences (preferences is what is normally started when you go to install a screen saver.)
Then, from the Apple menu or the prefs icon, you go to preferences / security, and there is no button. Just as I described. Pipes.saver is not installed. And prefs will not install it no matter how many times you try this. You can verify this is the case by going to Preferences, and then Desktop & Screen Saver, and looking at the list of available savers. Pipes.saver is not there.
Okay, so that's the OS install behavior as it stands today.
Now, take the Pipes.saver file, and drag it using Finder into ~/Library/Screen Savers
Now again, open preferences / Desktop & Screen Saver, and look at the list. There it is. If you choose it, it runs just fine.
This concludes our demo of macOS Sierra refusing to install working software from non-appstore vendors.