Rube Goldberg? Just because the factory didn't offer it, doesn't mean it's a cobble job. :p
The 3 gallon tank I have is for automotive use, as is the pump (both are even US made). Imagine having not had to take your front wheels off to refill them? Or having an impact wrench running off the tractor if you really did need to fix a flat? I'm going to be able to blow off the sawdust from my pants in the woods rather than tracking it back into the shop. The pump is small, not much bigger than two fists - it'll handle light tasks. I'm not putting a semi's air compressor on the tractor.
The additional hydraulics is just utilizing the fluid power available. Having a grapple thumb is 3rd function, but what if you wanted to run a sweeper and tilt it side to side? There's the 4th.
I've even looked into which pump is used on my tractor, because it might be possible to upgrade it without too much expense. 11gpm is fine for most stuff, but to run a sweeper or flail mower, higher volume is needed.
Didn't you know? I can't leave anything stock. :D