The Nomad is designed to get in, pick up the damaged ship, and get out. It can then go into orbit someplace safe to do enough repairs to allow the ship to proceed under its own power to the nearest starbase/spacedock for complete repairs. Basically, it's a warp-capable "band-aid" for starships. This gives Starfleet the chance to try to save the ship, rather than abandoning it in space. The Nomad can also pick up ships that are critically damaged, but that might have salvage options available.
I decided that some sort of "lifting equipment" would be needed in the drydock, and that not everything can easily be done with tractor beams. Besides, to access every area in the bay, you'd need multiple emitters, and they become a pain in the backside at times, especially when there's a ship docked for repairs. That effectively cuts off half of the bay's possible emitters. They're also a big drain on the ship's power reserves.
So, the Onge has a couple main emitters for assisting in the docking of disabled craft, but the other "moving" and "lifting" is done by the cranes. You can also think of them as "manipulator arms" like the space shuttle, for moving things around. In this case, having to replace a complete warp nacelle alone is a huge task, never mind the damage to the saucer section. "Lifting" the warp nacelle into position is difficult enough with the mass of the nacelle, but it also has to be held in position for fitting. Hence the use of the crane.
There's no reason why emitters can't be in the ends of the cranes, but they'd be used more for keeping things in position than moving something. In theory, with the way the cranes are mounted, they don't pivot - they're placed as if they move back and forth on tracks under the lip of the drydock. I placed them as if they are the gigantic overhead cranes you see in Earth-based shipyards. They move back and forth and pretty much just lift and lower. There's no pivoting. Therefore, the emitters would be straight line at the tips only, either outboard or down, with no "arc" for moving things.
With the way the Payne will be docked, one crane (port side) will be in place over the ship, getting ready to start repair work. The starboard side crane is in its stowed position, forward and almost against the aft bulkhead that forms the aft walls of the living/fabrication areas of the ship. If needed, the starboard crane could slide down the rails and be moved into position from that side, to provide additional support for the repairs.
Drop_Bear: Have at it! This is something that struck me one day while I was at Michael's, and I saw the UDT Boat kit. I'd never really thought of building something like this before I saw what became the donor kit. I'm making this up as I go!
Also, yes, the "cranes" are actually catapults, as in seaplane catapults. I wanted something that looked like a crane structure, but in 1:2500 there's nothing that would work that wouldn't be so fragile it would break when a mosquito farted within five miles. So I went looking on eBay for PE sets that would work. I was incorrect about the catapult source, though. The "cranes" are from the Hasegawa 1:700 WWII IJN Imperial Japanese Navy Battleship catapult set. The smaller parts mounted to the sides of the bay are from the Lion Roar LE700161 WWII US Navy CG Catapults set. I bought the sets back in March, so I had to go back through my eBay purchase history to find the details about them.
