This depends heavily on the definition of "useless".

The thought "I will not purchase this useless thing" is a thought about a useless thing, and it is not a useless thought. His formulation ("not necessarily") means that technically, it doesn't depend on the definition (given that you accept the previous example, of course).

If I have multiple trees it might be better to get all the low-hanging fruit before I move on to the higher level fruit in any tree

I actually parsed that quote as "Eat all the low-hanging fruit (in the orchard). Then eat all the fruit (in the orchard). Then eat the tree(s)." Well, not specifically thinking orchard, but I imagined running along a row of trees plucking all the low-hanging fruit, then returning for all the fruit, then shrugging and uprooting the trees.