Again, depends on what the project is.
That is the main consideration. A person making small boxes would have a problem trying to work an 8" piece of wood with a long jointer. Likewise trying to true the edge of an 8' plank would be folly using a #4.
Maybe Mr. Sellers is well practiced at using a #4 to get a straight edge on a long board with a small plane. For me, it is easier with a longer plane.
He does have a point about that most of the second hand ones are almost never abused so maybe that's a sign of less use?
If a person hasn't seen longer planes that have suffered abuse, they haven't done much looking. Is it possible in Mr. Sellers lack of interest toward longer metal planes he walked by them on the shelf without looking? Maybe the previous owners thought highly of their long planes and took good care of them?
After reading the article it appears Mr. Sellers has a different opinion on a long plane's usefulness if it is made of wood. A lot of bevel up users would also take issue with his view on their lack if usefulness. He does seem to think a #6 sized plane is sufficient. Patrick "I've never found this size plane useful" Leach seems to think the #6 is a plane looking for a purpose.
BTW, my first #7 was purchased from a woman who said it belonged to her father. He kept it in good order since he worked in a cabinet shop. It was a type 11 in great shape other than the tote and knob being painted blue for ID in the shop. BTW, the tote had been repaired.
After fixing up an older #7 the type 11 was sold.
When the metal meets the wood, it is all up to the individual user to decide what works best for their situation in their environment. My opinion, Mr. Seller's opinion, Mr. Leach's opinion are all just that, opinions.
Along with other's opinions there is often techniques, ideas or even some entertainment to be had by paying attention. What they say about a particular plane or tool may have some influence on my decisions, but in the long run I like to learn from my own experience seasoned with the experience of those who have traveled this path before me.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)