Spammers, trolls, and ban evaders often use temporary email addresses. PieFed now checks a list of known temporary email providers and displays a warning icon next to registrations that use such services.
If registration mode is set to "Open" (no approval needed) then the site admin(s) receive a notification instead.
A throwaway email address isn't always a bad thing but it's one factor that admins might want to take into account.
The more important demographic to use temporary email addresses are people who think their identity is not your concern.
Ah, OK, that's mentioned.
IMHO something like WoT in Freenet would be better. Having many cryptographic identities, but to start using one you have to spend effort confirming it's real. Solving captchas or whatever. Of course it's an obsolete solution, captchas are now solved by bots easier than by humans. But you get my idea.
There are competing concerns. The instance admin is concerned with avoiding de-federation by keeping the amount of spam and abuse coming from their instance to an acceptable level, without burning out. Some people signing up are concerned about their privacy. (Although this doesn't actually diminish their privacy or stop them from using a throwaway email address. It does suggest that their application needs extra care, tho.)
There's a balance to be struck.
In this case I'm leaning towards the side who pays the bills and the one who decides which software to install on their server. Without getting those people on board there won't be a place to apply for an account with and then act all distrustful about.
Someday when I make my super awesome instance-chooser, maybe "Email address is optional" will be one of the filters. Along with "Bans for criticism of China".
That's exactly what it does.
What does it matter if there's a place to apply for an account if I can't use it? Better not to exist at all.