xhonzi said:

Why do you think that Yoda and the Emperor were never implied to have lightsabres in the OT?

Why do you think that changed in the PT?

There are many reasons Yoda didn't have a lightsaber in the OT: Plot-wise, he knew Dagobah to be a safe hiding place so he had no need to carry one. Storytelling-wise, it would have been a dead giveaway for this anonymous crazy swamp creature to be carrying a lightsaber (Luke: "Oh jeeze! I shoulda known you were Yoda! The lightsaber! Duh!") And just visually, Yoda would have looked ridiculous with a lightsaber so it was left out of any design consideration.

Similarly with the Emperor: he knows he is always safe in his surroundings, and the "reveal" of his powers would have had less impact if he didn't seem physically defenseless before.

My PT memory is foggy, but neither of them are in "always safe" surroundings, there's no mystery about their powers or identity (there was a mystery about the Emperor's identity to other characters, but never to the viewer), and looking ridiculous was no longer sufficient reason for excluding something from the movies.