There have been some awfully good mountain bikes over the years that have the same sized wheels on both ends , so I'm not going to say a 69er is a great thing necessarily. But the idea behind it would be along the same lines as front-suspension only. Relative to smaller wheels, bigger wheels make obstacles smaller, pure and simple. The disadvantages of a big wheel are increased weight and, all other things being equal, a weaker wheel. Whether a bike is less responsive in the handling department because of bigger wheels depends as much on the bike's geometry as the wheel size, in my opinion. I've ridden a 29er off road enough now to be convinced that a well designed 29er handles just fine, even on tight, technical singletrack.

But getting back to the 69er concept, the idea is that it's more important for the front wheel to make mincemeat of obstacles, and where the front wheel can go the rear wheel will follow, even if the rear wheel's a smaller wheel (as on a 69er), or even if it doesn't have the benefit of rear suspension (as on a hardtail). Whether or not you're better off to have a bigger wheel on both ends, or to have suspension on both ends, would be another argument altogether (I don't mean this to start a hardtail vs. FS war, however ). With a 69er, you'd retain whatever benefits a 26" wheel has on the rear of the bike, and in the place you need it most (particularly the element of strength). And you'd get the benefit of an obstacle-eating big wheel where you need it most, which is up front. In my opinion, another advantage of a 69er would be that the geometry would be inherently slack, which makes for more stability. This would be an advantage, of course, only if stability over responsiveness is a priority.

If you look at the design of off-road motorcycles, you'll see smaller diameter, super beefy rear wheels, and larger diameter, not-quite-as-beefy front wheels. I suspect the 69er concept was born there (Cannondale saw to this with a 24"/26" bike back in the '80's in fact).........Sorry to the OP for going OT, I just enjoy talking mtb's sometimes -