Thats your personal problem though.
Disagree. There are no justifiable use-cases for the majority. The occasional constructive user may have multiple accounts, but those are a very tiny minority. The rest are shit posters / account farmers.
What do you base this on?
I base that on the time spent moderating + managing hundreds of users in Bitmixer. I'm most certain that managers and moderators (those that are tackling spam in one way or another) are the most qualified people to talk about this situation, unless there are other individuals that are somehow involved (similar amount of time/effort).
Considering that you yourself post sometimes 400 posts a month, you could easily split this among 5 accounts and enroll in 5 (different) signature campaigns. The number of accounts do not make a spammer, the post quality does.
Good luck finding people that are willing to spend significant effort in a similar fashion.