Wrong. Set up your GMail account in Mail normally, then go to Preferences --> Accounts --> Advanced tab and select "Don't keep copies of any messages". Make sure while on this tab to check the IDLE command so it will push email while online. Now, you have turned Mail into the ultimate GMail browser. The only problems are you need to be connected to the 'Net all the time and you can't search in the bodies of messages. If you can live with these restrictions, fine. If not, then change it back.
I do it at work and it's made Mail a hell of a lot faster and more usable. The reasons; to make search work, Mail downloads, stores and caches every message from every label in GMail and stores all of that in a database on your Mac. This makes it too slow for everyday use. I have thousands of messages archived. Also, I have lots of labels in GMail that I can't get Growl notifications from Mailplane for (I can specify one label to index). Since Mail can detect when new messages come into labels and display unread counts, GrowlMail can report on every new message that comes in.
Whenever I need to search, I go to the browser and do it

. I am waiting for someone to write a plugin for mail that will allow search while doing this.
--Steve