So we already have that. What I mean is an additional layer by requiring email addresses that have a more cumbersome registration process (not all that cumbersome for a legit user, cumbersome for someone trying to flood an instance).
Anyone can make a couple hotmail (for example) accounts, but it's a bigger pain in the ass than going to a temp mail site and getting a ton of inboxes going. Effort v. Incentive is the idea here - if you're a troll just trying to disrupt for the sake of disruption, if it takes you 5-10 minutes per account creation attempt from start to finish, you're probably going to focus your efforts on something faster to spin up. A latched door prevents opportunists and all that.
Does it stop everyone/the most dedicated trolls? No. But it narrows the funnel.
Edit to acknowledge after reading more of this thread and thinking about it more, I've voted Nay - let's see if the instance CAPTCHA is a sufficient barrier for this purpose first.