Never thought of the weight of a huge cassette but it is probably quite a lot. With respect to the aesthetics, I agree - the huge cassette looks pretty weird (but I could get used to it).

Our road tandem we have a 2x11 Di2: 52t-34t up front and 11t-40t in the rear. I've gotten used to riding it as a 1x on the big ring unless I think we are going to be climbing below about 10 mph for a while. Then we drop down to the small ring but I have to let my stoker know because the drop takes a second or so where there is no resistance. We don't ride big hills like we used to so I don't need to use the front derailleur as often. So it is kind of a 1x with a bailout.