David Weaver
I suppose my case for a heavy plane (and it's not a need by any means, I could get on with a #4, 5 and 7 without any other planes just fine), is that the heavy plane moves through harder stuff very easily. They stand apart on the curly cherry, not in results, but in which is more comfortable to use. No matter the wood, you can stop and start the cut anywhere.

But it is expensive, and the relative difference is small in straight stock. And in rougher stuff, the result is similar, it's just easier on the user, especially in a heavy cut.