Are single-sided forks a gimmick? The first time I saw a Cannondale with a one-sided fork I was in shock &amp; awe.

But I got to thinking about it, and it seems to me like it's just a marketing strategy, touting the quality and mechanical sophistication of the brand.

It saves some weight, maybe. But it loses so much strength. How could it possibly have a better strength/weight ratio than a traditional fork?
I think that their main point is not less weight but more control.  Imagine the normal 2-legged fork: even though the two sides are connected to the same air spring, you could concievably temporarily compress one more than the other, and now your front wheel is no longer pointed in the direction that the handlebars are.  This can only happen if the bridge is flexible, and typical fork bridges are very strong.  

The Lefty avoids this by using a single stanchion, which can only move linearly, but now must be able to support twisting motion.  I do not know if Cannondale has succeeded in making it strong enough, but Leftys have been around for years, so it can't be too far off the mark.

And, note the many schemes for rear suspension.  There are many ways to suspend a bicycle; no single type is so good that all manufacturers must use it.  The same is apparently true for front suspension.