I've got an IGH wheel (7 speed Shimano, I think), I bought most of the extra bits to mount it to a bike but am missing one particular washer that helps the shifting function work, and that missing particular washer is no longer available. So the project sits, waiting for an appropriate frame to mount it to, and waiting for that particular specific washer to either be found or waiting until I get the patience and perseverance to fabricate or carve one of my own. But then I wonder if it will be worth the hassle of all of that, then I just give up on the idea and put the bits and pieces back into some random box or tub or bin down in the basement and hang the wheel/hub assembly back up on a hook or lean it against a wall in a dark corner and forget about it for another year or three. It was going to be a winter project commuter bike when I first came across the wheel/hub assembly at the LBS clearance sale, so I guess it really isn't worth the hassle and effort to finish said project all due to that one particular missing special washer.

So, in answer to the question "what I have to gain", my answer is not much apart from the pseudo-simplicity of maintenance and being seemingly easier to keep clean and for the novelty of it. Breaking it down to change/fix a flat is a fairly significant consideration to some, others just accept it and work with the situation, and to me it varies depending upon the day.

YMMV