I know this one is pretty silly, bu I just like sitting in chairs that have arm rests - for me, its just more comfortable. Its just a dumb personal preference thing.
Anyway - I whipped up some arm rests out of 1"x8"x1' hardwood boards (I suppose you could use anything, but I figured the hardwood would be stronger. I cut a 3/8"W x 1"D groove down the end so that it would slide into in the 1/8" angle iron on either end of the sofa. The angle iron pieces weren't really square, so it took a few passes with the table saw to get the right amount cut out to make it work
To provide stability and strength, I then used 4 steel off-set pieces to lock the arms rest to the steel jack knife hinge. Oddly enough the off-set pieces came from the cam lock kits I bought from Home Depot (or Lowes - I can't remember). Each lock came with 3 different lock pieces (long, short, and off-set). Since I reused many of the interior lock pieces from the original locks, I had these left over.
I sprayed the wood pieces with the same color Rustolem I used for my TV mod and sprayed the off-set pieces and screws black so they would blend in. I covered the top arm pieces with some fabric - though I may go back and put some padding in and re-do that part - I don't know yet.
The arm rests slide in snugly and they feel nice and firm - no wobble. You can really lean on them without worrying about them breaking. When you need to unfold the sofa into a bed, the slide out nice and easy and can be stowed away.
Here are some shots: