Imagine, for a moment, that you are the Chief of Starfleet in a post Dominion-war universe. You've got a bucket load of starships (in various states of repair), more starship designs than you can poke a stick at and limited production facilities.
Now the question: Which designs would you continue to produce and which would you shelve?
Would you continue producing light frigates, heavy frigates and fast frigates, or would you just have the one 'generic' frigate? And the same for cruisers, explorers and escorts?
While I understand that everyone likes designing their own ships, and Spacedock is a wonderful tool to do this, surely Starfleet are going to have to put their foot down somewhere. The practical problems of commonality of spare parts and personnel training would surely demand some sort of standardisation. ("Hey, sorry, Captain, I'm not rated on that Impulse engine. My last posting was on the USS Improbable.")
Please note that this is not a request for starship designs. Please only list classes that have been tried and tested through the Dominion war.
If this needs to go under the Spacedock section, please feel free to move and/or berate.
------------------
"You couldn't even SPELL Prime Directive!"
"Not true. It's spelt P-H-A-S-E-R."