While it might be nice to be able to pilot your favorite ships at T5, it doesn't make much sense.
A Miranda being able to go toe-to-toe in a fight with a Defiant or Sovereign? It should realistically be akin to smashing through a issue with a jackhammer.
Sure, and it makes about as much sense as Mirandas going toe to toe with battleships. And those battleships, in T1 on normal difficulty, being about as tough as wet tissue paper.
Now, I agree on the Miranda, but for a different reason. I'm not a fan of the T1 ships being used as other ships, as that leaves the issue of what type of ship it is. PvP exists in this game and a player should be able to know if your ship type is escort, cruiser, science vessel, carrier, etc from looking at it.
But beyond that, realism can stuff it.
If Star Fleet can make a nifty new version of the NX class, then they can do the same for the T3 cruiser. And, no, the ships should not be less effective. That would just mean very few people would bother with it, so why waste the dev time? If a ship the size of the Defiant or a BoP can be effective at T5, then a ship bigger than them can also.
It'd diversify the look of what people are flying at T5, something I think would be pretty cool and a bit closer to the DS9 battles. It'd let the fans of the various Trek series pick the ship appropriate to the Trek they like. And even ships from Naboo for fans of the Star Cruiser look. And I'd get to fly in my T3 cruisers again.
It'd also mean the freedom to pick a ship that has a variety of models to mix and match parts from.