My personal belief is that it is not critically necessary and more of a personal choice through a matter of convenience versus presumed need.
Once I have my pond establishment constructed, it will have two bottom drains which will gravity feed out.
Here is the reasoning and a little information.
My pond is actually a rectangular HDPE tank (10.5' x 5.5' x 28") which I am going to use as a fishing bait tank. The bottom of the tank will be resting upon a solid, flat wooden structure, not sand or dirt or mud and since the tank is hard-walled and not an EPDM liner, it won't be moving around in any manner. So I am going to install flush mount shower drains as my bottom drains, just like a regular home bathroom shower stall. This way, I won't have any obstructions within the tank and when I desire to retrieve baitfish from the tank, I can just swipe a net through it and the net won't get hung up on anything.
The bottom drains in my case serve a specific purpose... No plumbing or pump obstructions in the tank (pond) and they facilitate better removal of debris from the flat bottom.
If I were constructing a below grade pond with a liner, I don't think that I would install bottom drains. I would just place the pump near a low spot or provide some sort of pickup tube.
Gordy