Thanks for the replies and they all make sense: horses for courses, I suppose.
pe2dave, I think I'm in the same camp as you. If I were regularly being confronted with seriously rough timbers, then a (wooden?) scrub might be an idea but fortunately the rough sawn stuff from the local timber yard doesn't come into that category.
You'd still want a wooden jack plane rather than any supposed purpose made scrub plane. Scrub planes weren't used to dimension rough sawn lumber because they make a mess of it and take more energy to use. I have a surplus of jacks. How anyone is going to set up a plane like a jack is going to be based on what's about a brisk walk effort level.
I measured mine - the radius is hand cut, so I don't have a clue what it is. It's minimum effort level for hard softwoods through beech in medium hardwoods. When I make another plane and set the radius otherwise (narrower for "harder" woods or something where less cut is desirable, then those planes end up in disuse as it makes more sense to dimension just less cut on the main jack).
At any rate, what I found just now for projection from the plane mouth is 1.4" of width protruding from the mouth of my jack plane and the peak of the projection is around 5 hundredths of an inch. Common pitch bedding, so the radius is steeper than that by something like the square root of two.
Flatter than this, and removing wood becomes problematic. Less flat, and removing wood becomes problematic (due to tearout, separating of fibers, blowing out edges, etc, and leaving behind a surface that has unintentional deep spots in it - creating more follow up work).
I hand dimension about 300-500 board feet a year (rough sawn). That's not a huge amount, but it's enough to get a very good idea of what's less effort and what's not (when to resaw, when to plane, when to rip, etc). I would guess that I use this jack plane to get within less than a 16th of a marking line and then clean up to the line with the try plane (which only has slightly more camber than a smoother).
The guy who got me into woodworking likes the logical idea that a narrow short plane can really "spot work" on wood, and he loves the shavings that come off of the plane. I've seen him use it twice for wood that was too wide for his jointer (that was badly cupped or twisted). He made a mess of it and fortunately the wood was thick. To him, a jack plane is more like something more like a try plane or jointer, because that's how he'd use one.
I do have a power planer (but no power jointer). On the rare occasion that I use a power planer a couple of times a year, a wooden jack is good enough to prepare wood for it -scallops or not, the board is flat enough and doesn't bind, and a pass or two removes all evidence of the jack plane (just like the try plane does).
There's one other benefit of the wooden plane - if you happen to be working wet wood, it won't create too much friction. metal planes are terrible on partially dried wood.
(I suspect instances of older planes with really drastic camber - prior to the marketing of a scrub plane -we used to clean up wet riven lumber when the orientation on the face and the wetness make mass removal much easier).
