That's actually not true at all. If a hero has 10k net worth, those 10k are not split between all remaining players - you first have to sell those 10k in items the hero has, which loses half the value (so 5k in this example). Which means at the instant the leaver leaves, you immediately at least lose half their net worth, in this example that means a net worth swing of 5k in favor of the team without a leaver.

There's just simply no situation where having a leaver is an advantage, even for a short period. (Except of course if the player was actually having negative impact through griefing or similar, which would remove their negative impact... this might actually not be too rare...)