This helps protect me in many ways. If my email is sold or leaked, not a big issue, I can just add that specific email to a blacklist and I never need to get spam from it again. Or if I cancel and keep getting spam about rejoining, blacklisted. It makes it easy to keep my spam and newsletters to a minimal.
It has the built in advantage that I can always sign up for new trials if I want, just do thatdomain.com1@ thatdomain.com2@ and so on. Although I don't do this often, I have had to do it for various reasons.
I've hit on occasion websites that block their domain from being in the email address, likely a poorly implemented security check because their software might say anyone with a "@service.com" email is an admin or something. In that case, I enter some random crap. I never have to remember the emails, since I can just search my email history for the address the service sent the registration confirmation to.
However, the downside is privacy. I use my own domain, which contains my full name, so when I sign up to some services and want to do so without giving my name, I still rely on a disposable email service such as hidemyass.com; and I do this for many online services. I am not a believer that everything I sign up for needs to know my full name, address, and email - often services ask for this information for no reason.
So attempting to block these types of services, that have valid and useful benefits to users, simply harms your users. You can avoid spam users with a captcha, and for trail abusers you already can't do much because @gmail.com already allows for a lot of aliases to work like @googlemail.com, or user.@gmail.com or u.s.e.r@gmail.com etc, or user+whatevertheywant@gmail.com
Don't harm your users with useless validations.