There are advantages to a singled truck, and some disadvantages as well. As you stated above, there is less weight, less tires (well, same # tires if you go back to duals) less drag, etc, as well as not falling under the 3 axle rules. You lose the 2 axle interlock capabilities, but if you have a locking diff, it won't be an issue.
Mine was singled short, and I wish it was still a twin. I am not just using the truck to haul a single crawler though. I pull a 48' drop deck with it occasionally, and wish I had the extra weight capacity, as well as the interlock. (mine is not locked, so it gets stuck on stupid stuff) I have been looking into a frame extension and an intermediate take-off axle to put mine back to a twin, but not sure its really worth the effort.
A lot of the conversions will drop the front axle and move the rear axle forward. If it is singled short, the axle moves to the original front axle location. Singled long just leaves it in original spot. Singling it mid splits the difference. Most opt for this location because it makes it look more proportioned. It also takes some of the weight off the front with a bed on it vs singled long.
I agree that they look goofy singled long, and also think they look funny if singled short without bed modifications. Mine has tool boxes behind the axle (in the location of the original rear axle), which makes the overall length about the same as it was stock. When hooked to a regular trailer, it almost looks normal. When bob tailing, it still looks a little goofy, but not as bad as it would with a short bed. Just my .02. You can really see the difference with something loaded on the bed.