if you think that rMBP is too heavy, a monthly gym membership would be a better choice than a rMBA
Very true. The rMBP blurs the line between ultra portable and high performance laptops. I'm currently using a 2008 Macbook and it's heavy enough to be obvious when carried around from place to place. Sure it's not too heavy, but it certainly adds some noticeable weight to my work bag.
The 11 inch Air still holds the crown for portability. It's really light with a small footprint not much bigger than a full sized iPad. Sure it isn't going to play top end games or render a large 3D model, but then that isn't its intended use. As far as a miniature fully functional Mac computer goes it ticks all the boxes.
However the 13 inch air is close to redundant now the 13 inch rMBP has been released. I have had the pleasure of using the smaller rMBP and I've held a 13 inch air. The Air has a larger footprint and there isn't enough in the weight when you take the feature set into account. Right now I wouldn't buy a 13 inch Air because price for spec it isn't worth it against the rMBP.
After 3 years of the same design and the introduction of the 13 inch rMBP pro I think Apple is probably preparing to redesign the Air to address this. Especially if they intend of making the rMBP even thinner as recent rumors have been suggesting. Maybe not this WWDC, but certainly in the next year or so I would expect to see a slightly smaller, thinner, and more lightweight 13 inch Air.
Maybe it will be this WWDC though. By the sound of recent rumors they are only going to be releasing the new Macbook Airs. That could be a marketing move to allow a new design some time in the limelight before showing off a thinner rMBP.