I'm also elated for this. Countless times at work, I send an email from gmail using my work/school .edu email and when that mail gets replied and forwarded around back to me with my gmail user name exposed. Not a lot of people know me by my username (which is my gmail) and sometimes I want to keep it that way.
So you don't want your username exposed. On the other hand, some people do not like to reply to Gmail addresses because they don't want their mail to be scanned by an advertising company.
Forwarding email from a random address to a gmail address has been possible as long as gmail has existed. It's not a feature that depends upon Google support.
