In this scenario you are the somebody else hosting the exit node. For the anonymous strangers using the exit node you are hosting for them, they benefit from having their traffic mingled with yours because theirs won't stand out as much. For you, you benefit from having the anonymous strangers' traffic mingled with yours because anyone spying on you will be very confused.

My guess is that a professional analyst is not going to be particularly confused by the addition of a single user's non-Tor traffic to the firehose stream coming out of an exit node.

But that's just my guess. Perhaps somebody should ask @snowden.

You've got the proportions backwards. A typical Tor exit node only provides a few hundred kib/s of bandwidth, maybe a couple of mib/s for the more generous one. Your traffic, or even the neighbor mooching your WiFi, will be the firehose stream in comparison to that.