Novelty / Practice
I'm kinda fascinated with the concept, but I have to agree that most of the value is novelty -- that, and the opportunity to practice making tools and working with your hands.
I'm trying to imagine a scenario where I'd have what I need to make that knife, yet not have access to 1) other, more suitable tools, or 2) an actual knife.
If I find myself adrift in post-apocalypse MetroTown, and I'm wandering around an abandoned hardware store, it's very likely that I'll find way better stuff than hacksaw blades.
If the scenario is "all I have is what's in my toolbox" then I'm good to go -- you have any idea how many knives are in each of my toolboxes?
Scenario: stranded with only what's in my car? Uh, sorry, there's no hacksaw in my car, but there are at least three knives in every car I own.
Scenario: stranded without the car and only what I'm carrying. *Checks pockets* Nope, no hacksaw. Five knives, though, if you count my Leatherman wave.
Any scenario where I'm compelled to make a knife from a hacksaw blade is gonna be terribly contrived.
However.
Knowing how to make any kind of knife at all is a skill worth having.
If I can make a knife from a piece of random steel that's badly suited to the job, there's a good chance I'll be able to adapt something else when the time comes.
Skill is the ultimate in non-depreciating portable currency.