I never said in my posts that there was a sameness in working out with the punching, double-ended and/or heavy bag. In fact, when I wrote:
I'll agree that, by itself, that's not earth shattering. Add the double-ended bag (An even more mobile target) and the heavy bag, and you have some worthwhile skill-honing going on.
I meant they complement each other.
My question was directed at the methodology many boxers use to hit the striking bag. I wasn't concerning myself to the double-ended bag or the heavy one, except as a passing thought.