You hear stories sometimes. About how when a deliverability person warns sales that they shouldn't sign that client, but the client comes aboard anyway. "If they do bad things, so what? It shouldn't impact other clients. We'll give them their own domain, their own IPs, and it'll be fine."
Are you sure?
Too many times now, I've seen blacklists like SORBS or Spamhaus blacklist whole ESPs or whole large blocks of IP addresses at an ESP. I bet it's not fun explaining to client #2 that their bounce rate jumped to 50%+ because of the bad acts of client #1.
And this isn't just something that happened in the past. Just about two weeks ago I saw Spamhaus blacklist 255 IP addresses at a particular email service provider due to the actions of a single client. (The listing is since removed, so I can't link to it. And my goal isn't to name-and-shame, so I'm not mentioning which ESP it is. If you're smart, maybe you can figure it out.)
You might argue that Spamhaus appled too broad a stick and perhaps they shouldn't have done that. You might be right. Complain all you want, though, but you can't control Spamhaus, and neither can I, and neither can that ESP. But that ESP can control what clients they allow to use their services, so I would argue that they did have a way that they could have prevented this.
Assuming that one client's bad practices won't affect other clients is a risky proposition.
And I'm not even touching on what this kind of thing does to an ESP's reputation. If you want to be a member of M3AAWG, or if you want ISP people to respond to you favorably when you sometimes ask for help out of band, you need to not have the reputation that your platform will take any client, even ones with bad practices.