It is a valid approach and good idea. But also ppl who do it should know... it ties all the accounts together through a domain no one else uses. With a privacy mail provider there could be millions of others on the domain. With a prvate domain, not so much. It's one more piece of data that helps identity brokers deduce that account A on site A is the same person as account B on site B.
Some might not care about it. Or, they might. Threat models etc.