The number one item on Twitter earlier was offline Gmail... I just have to ask, who cares?
I have offline Gmail, it's called IMAP. Windows Live Mail or Outlook downloads all of the messages locally and it doesn't require some random custom web server running in the background on my machine. This is how I back up my Gmail to a local disk in the event they lose all of the email (deleting the account, etc. It happens.)
Here are the upsides and downsides for both parties:
Upside for you: you can use the Gmail web interface if you prefer it and still read msgs offline.
Downside for you: you have a random service running on your machine that hosts a web server.
Upside for Google: you continue to use the web interface all the time, therefore placing their ads in front of you.
Downside for Google: none.