I believe that it will cause problems like this in practice:
If you have 5 fleets, and 2 admirals that give some fleet size increase that you're actually making use of, then whenever you re-assign these admirals to a different fleet, that fleet will split, and the fleet the admiral was assigned to now needs more ships assigned to it as well to actually make use of the admiral's skill. If It switch the admiral back, I have to merge two fleets.
I reassign admirals for various reasons, for example to help a slow fleet catch up with the main fleet (if I have admirals with speed increases), or to give a bit of extra combat power to a fleet about to start combat (like in the separate my doomstack and take systems phase of a war).
Also, if a fleet split happens due to an admiral dying or being reassigned, will the excess ships continue to carry out the same order they had?