Spock, there would be the usual argument about compression of a long length of spiral-wound wire outer versus the better sealing it affords. Sidestepped neatly by gore or goodridge cable sets which use the cable stops on your frame and seal the inbetweeny bits with a slim plastic hose, theoretically giving you the best of both worlds. Provided it is well installed. I can’t say my goodrige set has really made it that much better.
I have also heard on here (but never seen) that a rear mech snapped off and tangled in back wheel at speed will pull the bars round if it is connected by full outer.
And no i don’t think anyone makes a widget to put cable stops in. (though you can get thingys that do it the other way round. You could get them welded on if you are really that bothered. Personally on a bike with no cable stops I would go full nice quality outer with 1:1 cable pull shifting (ie sram or 10-speed shimano).