Just ran a simple test - the swf for a practically completely empty Flex 2 Application comes to ... 116K
! Wow. Then again, adding components to it doesn't add all that much, since practically the whole mx framework lib must have been included already
So, if going for a big application, the weight will probably amortize somewhat. But I guess there is just no such thing as "simple and light" in the world of Flex 2.