They are for whichever use the customer wants. When used in outside facing doors (or doors you want to actually lock down) it should be paired with a deadbolt, as it is here. You are not wrong in saying that the main reason to use this solution is cost tho.

The reason they don't really use a single spindle anymore is because it makes them extremely easy to brute force. You can literally just knock the handle off and access and operate the latch assembly.

Those locks dont have a locking mechanism (hence the deadbolt) they are just meant to keep the door shut

There are some that do lock the latch. In those cases removing the handless wont do you any favours since the assembly itself is locked in place. And there are others that dont lock the latch but have a deadbolt as part of the lock.