To me, the bag that is being used in your photo is more functional than a Tailgator. I've ridden brevets with both a Tailgator and a Detours High Tail (they now only sell the expandable one: http://detours.us/store/seat-post-bags/high-tail-exp/). I preferred the Detours for several reasons: the Detours opens like a clam shell, but the Tailgator opens only from one end, which makes accessing gear more complicated. This is especially important for me on long rides or on the trail, when I am tired, dirty and want to keep moving -- having to rummage around or unpack a bag is not ideal for me. The Detours is also more water resistant (the Moots, at least the iteration I had, is only treated cordura as I remember) and includes a fitted rain cover; the Moots is/does not. Finally, the Detours uses a type of clip attachment that means when I am not using the bag, there is only a small clamp remaining on my seatpost; the Moots requires the seatpost to be removed to get the entire rack off. While the rack is very light and elegant in its own right, I didn't want it on my seatpost when I wasn't using the bag.
All this is not to say that the Detours is right for your application -- as I said, personally, I like the bags shown in your picture (the name of which is not coming to me right now) -- I've never used on, but for a number of reasons they look to me like better designs for trail use. I'm only using the Detours as my point of comparison for my own sense of the Moots -- nice idea, very nice execution in some areas, but for me, ultimately not the right solution at all (that said, a friend of mine has my Tailgator and uses it happily).