1. Some users may provide a real email address instead of a throwaway Mailinator one. The owner of the site can then send email to that address (which might get read and result in money being spent) or sell the email address to some third party.

Now, someone who would prefer to give you a Mailinator email address probably isn't likely to buy whatever you're spamming them with, so the benefit is probably pretty small. But it might be nonzero. (And if you're selling your mailing list to advertisers, of course they won't know how many of the addresses are for people who will ignore their spam.)

2. Requiring a real email address (that takes some effort to acquire) makes the cost of abuse slightly higher. So if you're offering a service people might want to abuse, you may reduce the frequency of abuse a bit by forbidding throwaway email addresses.

I did understand point 1: but this type of block would illustrate that a primary concern is the ability to abuse my personal e-mail address. Assuming a company doesn't have that intent, I don't think they should send that signal.

Point 2 regarding abuse in the other direction is more interesting, and a fair point, thanks.

In my experience, Mailinator addresses have only been used as a way to abuse our service or try to keep getting new trial accounts. Of course we have mechanisms to protect against that, but it's still a large amount of cruft we have to deal with and clean up after. The number of paying customers that use Mailinator or started with Mailinator and changed their address afterwards is zero.