I don't know if they are "worth" the premium, but we pay it for the convenience. We stayed at Grand Flo for our honeymoon, and it was awesome to be right on the monorail. We are going back in Feb/march with my in-laws, and are hoping to stay at bay lake towers. Having a pre-schooler and a baby, the convenience of the monorail is just too good to pass up.
I have only been in the 3 monorail hotels, so I don't really know what the others are like.