This opinion comes from the fact that Yoda and the Emperor are both considered the masters of their particular side of the Force, and as such their powers have grown beyond requiring weapons. Look at the poor opinion Palpatine has of lightsabers when he handles Luke's. In fact, I'm pretty sure Yoda was never meant to have a weapon in the original vision because his philosophy is all around Jedi using the Force to seek knowledge and for defense but never for agression, and saying you don't need weapons.
Vader was never considered a master, he was an enforcer, started his role as Tarkin's big mook or dragon, and graduated into a much more pivotal character once he was more fleshed out, but he needed a saber because he was the enemy of Obi-Wan, who used a saber, and Luke's, who followed in his footsteps.
As much as I despise the prequels, I think the Palpatine vs Yoda scene would have been FAR more epic if it hadn't included any sabers for either of them. Just have them use badass powers, like Yoda does when he enters the room and slams the Imperial Guards against the wall. Make the scene shorter if you have to, but those silly saber moves were ridiculous being used by old people doing flips and shit.
