Why a "derailuer" type chain tensioner on an IGH?
 

The post says it all. Why would a bike like the giant transcend pictured below, have a chain tension-er on an IGH hub?

http://www.giant-bicycles.com/en-US/bikes/lifestyle/

Pardon the typo on "derailleur"

That is a really good looking bike. I would want one if I were in the market for a new commuter.

The chain tensioner could be used to allow simpler cog changes on the rear hub, as the longer derailer shape would be able to handle a wide range of adjustment.

Also allows for slightly easier wheel removal, I would think.

Likely because the frame is designed as a derailleur bike with vertical drop-outs so the derailleur has been kept to tension the chain rather than change the drop-outs to horizontal. A bit hokey if you ask me.
Edit: I see it has disc brakes. The vertical drop-outs are there to keep the disc rotor aligned with the caliper, making an alternate form of chain tensioning necessary.

I agree with previous posters about the need for the horizontal drops due to the disc brake torque. However wouldn't a single pulley tensioner be enough?

I figures something along the lines of needing the chain slack to get the wheel on and off, but I did notice that other IGH bikes didn't use them. I couldn't see the drop outs in the photo to make a guess. Check out the Specialized San Francisco, no tensioner. But alas, 26 inch wheels and a heftier price tag.

http://www.specialized.com/bc/SBCBkM...co&language=US

So far, the Giant Transend is looking like a match for my next bike purchase. I wanted IGH, disc brakes with 700c wheels, and enough clearance to be able to fender it with studded tires. Such a bike would match my winter riding needs rather well. The trek soho also caught my eye - belt drive? Looking at the rust developing on my winter beater bike chain makes me want to take a look at the belt drive. I also never tried the shimano roller brakes - a google search doesn't turn up many favorable reviews of the front roller brake.

The bike is using the Shimano Alfine IGH drive train and the associated Alfine tensioner. I would guess that a number of single pulley tensioners would accomplish the same thing on these bikes, but but a double cog tensioner can have a couple of advantages: It can bring the chain up a little higher, improving chain wrap, which can be essential if using a small cog on your hub. Also you can pull more chain with a double pulley system. Not only will this make it easier to swap out cogs of various sizes, but it makes it possible to run a front derailleur.

So on those Giant bikes, I would guess that they have a tensioner, as has been said, because they have vertical dropouts, and the reason they have the double pulley, Alfine version is just because they have the corresponding Alfine hubs.

The Swobo Dixon and Civia Hyland both have disc brake gear hubs and solve the disc to brake caliper adjustment differently so there are alternatives.

On the Swobo the caliper mounting holes on the frame are slotted so that the cakiper can be moved when the chain adjustment is done to keep the brake pucks over the disc. This idea is also used on at least one Surly frame. The Civia Hyland Rohloff uses adjustable semi vertical dropouts and mounts the caliper to one of the dropouts so everything moves together.