Gmail and others allow you to "make up" From addresses. I never send from my gmail account address. I have made a SG address of email@example.com
and made that my default sender address. Gmail and others do not care if you designate this as your official sender address, just as long as it is a valid address (they do the test email for verifying). I have about 4 SG addresses for this purpose and choose them at will. It works fine and then if someone replies then it still comes through SG.
I do not see a reason for SG to provide SMTP services and such. The only improvement I could ask for is an easier way to manage my mass of accounts with a way to search for accounts getting low on clicks.
As for companies that "block" SG accounts, I dont need them! The only reason they are blocking SG email addresses is 'cause they want to be able to abuse your email. Pure and simple.