To be honest, it's inefficient to not upgrade the Jungle item BUT, not immediately and eventually the Jungle item falls off.
Think about the costs of the item, because that's what matters.
- So you have 350g for Hunter's Machete, which both Junglers pick up.
- One Jungler will finish the item at 1,000 (650)g.
- The other waits for another 550g for Tiamat.
Now think about who is stronger? A Tiamat or a no damage Jungle item? Obviously the Tiamat is more powerful right, but the other Jungler has an advantage in Gold because his original item isn't going to waste. This translates to a much faster first item.
- Now 10 minutes in, the guy with the Jungle item has a full Warriors Enchant
(or any Jungle item, they all cost the same).
- The Jungle with the Tiamat only has this:



Now assess the situation again, who is stronger? Well it honestly depends on the champion but by items alone, the red smite damage and +10 AD from Warriors is going to be a lot more powerful than the Tiamat and Lethality. This is likely even being a bit generous because I can't see most Junglers even making it to Serrated Dirk. There's a good chance it would just be two long swords and some extra cash by the time the other Jungler has finished their full item.
Taking a regular item start as a Jungler does come at costs. Everything mostly comes down to how fed they can get early, to make up for the gap between them and the other Jungler.