Hi All from sunny but cold New Zealand.
The throttle cables were always stiff to return on my 1988 Concours. I had them off while installing new carby boots (purchased from Murph's and real quick delivery), and found them individually free but collectively stiff. They were lubed with CRC and had no obvious kinks or had been mis-routed. I tried a heavier return spring - slightly better return, but too heavy to twist on, especially when on a long trip.
I have just recently installed Avon heated grips. The plastic twistgrip was quite brittle and shattered while I was shaving it down to take the heated grip. Local Kawa would have to get one ex- Japan. (Try getting stuff out of Japan at the moment- half the warehouses written off by the tsunami.) Found an aftermarket motocross throttle-grip assembly of similar proportions and proceeded to fit it all up. Well the proportions weren't quite that similar. The location holes for the cables were at such a different spacing I could not get the slack adjusted out of both cables. Sooo as a temporary measure, I left the return cable off. The throttle is now much lighter to twist and springs back when released. I really like it's response. I talked to fellow riders who suggested the second cable was a safety feature and should the pull cable break, you can still push it off. I figure if the single cable is going to wear, you will feel it starting to drag before it gets to the point of breaking.  There is also the kill switch and ignition switch after that, if the cable ever did jamb. Interested in your thoughts...
cheers
tcbob 

fix the thing correctly. I have had the return cable snap before, and it causes the grip to over rotate forward, this can cause problems in the pull function by jamming in the housing, or even causing a jam at the carb attachement points, with catastrophic effect....
don't take chances with throttle or brakes....it ain't worth it.
You can find that part on ebay for a couple bucks.

fix the thing correctly. I have had the return cable snap before, and it causes the grip to over rotate forward, this can cause problems in the pull function by jamming in the housing, or even causing a jam at the carb attachement points, with catastrophic effect....
don't take chances with throttle or brakes....it ain't worth it.
You can find that part on ebay for a couple bucks.

+1.........Do you feel lucky? Not worth the risk to me. My experience is stuff always goes wrong at the least opportune time. You may not have time to hit the kill switch.