Star Trek III screwed things. The original idea for the heavies was Romulans with their Romulan Bird of Prey. But then, they changed it to Klingons, but kept the BOP concept and the cloaking device.
In TOS, the Romulans were nothing special until they got cloaking technology, which tipped the scales in battle. To prevent their technology from falling into enemy hands, they self-destructed their ships. But now that Klingons have the cloaking device, it hardly makes sense anymore. Also - where's the honor in tricking your enemy and hiding?
It's one of those things you just have to handwave. After all, if cloaking technology is everywhere, why doesn't everyone use it? Sure, the Federation signed this "treaty", but how come the Cardassians, Dominion, or Borg don't use it?
1) The Romulans had cloaking technology from the first episode of TOS onward and evidently they gave the Earthmen a run for their money during the war.
2) The Federation does have cloaking technology. We saw both cloaking suits and the cloaked holo-ship in Star Trek: Insurrection