I know from experience that the idler gears in "some" Indian makes are just plain crap. The whole gear train in the engines has proved to be a weak point and until we can get the Indians up to par on the backlash issue, it might be advisable to have the easiest to replace gear made of a softer material. After you have changed a cam gear or two anyone will understand that. All of my engines have bronze idlers as standard and I suppose they could well be classified as "wear parts" to be replaced on a regular maintenance basis. Having said that, after 2500 hours on a 6/1 and 2400 hours on a 20/2 I have not seen signficant wear on my idlers, so for the "occasional" user it is not going to be a significant issue. Yes, there is some "gold dust" in the sump of one of my 20's indicating that the gear is wearing slightly, but as I am aware of it it is not a problem. Busted iron idlers most definitely are, and hardened ones will only serve to destroy other components such as the cam gear and the crank gear. Personally, I'd rather not go there.
I think the main problem is that the hole for the idler gear is slightly off in some engines, causing backlash problems. No aftermarket idler is going to solve that and a hardened one is merely going to ensure that other parts that are much more difficult to fix, break.