You are correct that items of similar quality perform so similarly as to be indistinguishable from one another in practical use cases. That, however, is not the issue...the issue is player feeling. When it comes to player feeling there is a clear preference for improvement in any game that uses items and item statistics. As soon as numbers get involved humans, and by extension gamers, want to see to those numbers go up over time. As soon as we get the sense that our numbers are stagnating or that there is no chance to improve them, the fun factor diminishes.
Let's assume that you are correct in your example that you will return to 72 damage from 70 in relatively short order. How does that feel? Not like progression, because you've now spent X amount of time/effort just to get back to where you already were. That's the opposite of fun, IMO. Fun comes from feeling like you are getting better and moving ahead, not treading water. Even if that feeling is illusory because the actual difference in numbers is miniscule, that doesn't change the impact of the feeling on your gameplay experience.
I would rather keep hunting in the hope of finding upgrades rather than praying I've stockpiled enough backups to avoid losing ground. Maybe that's just me, but I don't think it is.