No big deal.
Some months ago, I felt fed up, got angry (no meds), and decided to quit playing, then I gave my tablet to a cousin. He asked me if he could play using my DF account, and I said "why not?"
I just changed some settings (google play password, for example), of course, and gave him the thing.
He played for some werks, and then got married and moved to Mozambique (work), and for what I know, he stoped playing. Wives...
But thing is: I know he shown the game to his friends, and many of them are playing the game right now, because of this sharing-publicity. You welcome, Zup...
About the original post`s question, my humble opinion is: I couldn`t care less, cuz if the 2nd player is better than the account owner, he is wasting his time (cuz he could use his time on his own account, huh?); and if the 2nd player is worse than the owner, the owner is wasting stats.
And after all, as Octavia and Dognammit said, there are tons of good reasons for sharing an account.
I, for example, wont let anyone play for me, but I dont see any wrong on it. No cheating, no cheap play.