Can I copy (myself) an app purchased through the Mac App Store to another of my licensed computers? I have some applications purchased through the Mac App Store, and I have two Macs.

Presently, if I buy an application on my Mac mini, I just go and launch the App Store on my MacBook Air, and then download the application again from "Purchases".  And vice-versa if I bought on the Air first.  Similarly, I apply updates for each computer separately. I expect this is how most people do it.

However, this practice consumes bandwidth. For small applications, it isn't a problem.  But what about big ones?  Say, the new Xcode, or a large game?

What I'm getting at:

Is it legal and is there a way to copy an application, myself, from one of my licensed computers to another of my licensed computers?  By "licensed", I mean both of my Macs access the Mac App Store with the same iTunes account, and therefore have access to the same apps, anyway.  I'm looking at reducing Internet bandwidth consumed, not circumvent license restrictions.

Might a simple file copy of the application folder work?  Or, is something like this not supported and I just need to accept that buying apps this way will necessarily consume N * download size worth of bandwidth for each purchase and update?

Consider, with iOS devices this isn't an issue, as long as you update one device, sync to iTunes, and then sync the other devices to iTunes&ndash; they'll get the updates without re-downloading.  How to do something like this for Macs?
Yes

You can simply copy the application to your Applications folder, and then login to the Mac Application Store with the same username and password. This will validate the application and allow you to run it without needing to download it again. 

The only time this will become a legal issue is if you are copying it to someone else's computer, however, they will not get updates for the application unless they log in as you, or otherwise, unless they also have a purchased copy of the application.

You can also read my answer here for a more detailed explanation of Application installs on the Mac and the Mac App Store.