Oh, the threat isn't to the user. They don't care about accounts they have abandoned. The threat is to services that suffer vote manipulation, spam, etc. The benefit of this approach is that you've got a bunch of abandoned accounts that were organically created (captchas solved, residential IP ranges, etc.)
You can get all the stuff you mentioned for a fraction of the time and money it would cost to create a disposable email service and then you would also only target the service you wanted instead of sitting around and hoping that a user creates an account at the site you're looking for.
The real value would in the reputation and history of the account (e.g a popular account on a social media site or community) and this isn't a thing that someone would use a disposable email service for as they wouldn't want to risk losing access to the account.
