FormationFlyer
You are absolutely right about it being possible to hold a heading precisely; the difference is that on an ILS you only have to do it for a few minutes at most, and you have done your checks well before you get there, so you can concentrate on the job. You also have a clear visual reference: the CDI or the HSI bar, plus the heading bug, most club planes don't even have a heading bug. I doubt many people can fly better than +/- 5 deg en-route and if they did they would not enjoy the flight very much.