That's really unfortunate. It seems like a big loss of privacy with weak arguments in favor.

(1) They could easily spoof one UA for mobile and one for desktop -- user chooses.

(2) Why are they worried about the experience of Google Docs over Tor??? More generally why focus on the use case where people enable all kinds of unsafe javascript to do fancy OS-specific things?

It's like advertising your lightweight speed-focused track racing bicycle, then putting massive tires on it because some users had trouble going off-road.