But if you do it really early the extra turn means so much less. And extra turn later when your deck is much more productive seems a better use of the extra turn, but i haven't done any analytics on this, just my gut.
It literally doesn't matter how productive your deck is. Dominion is a race to ending the game with a point lead, and regardless of when you take an extra turn, it gives you the exact same amount of extra time to do that (a turn). Even Seize the Day works like this, but even though its effect stays the same throughout the game, its
cost becomes less and less significant as your turns get bigger, which is why it's generally best to buy it late unless you happen to have $4 and a buy you would otherwise waste or spend on something inconsequential.
What does matter, however, is that Dominion has player interaction so you might have a reason why you want to take two turns in a row without letting your opponent do something in between them, like buying the last City on the normal turn and then using the extra turn so that you can also be the first player to benefit from Cities being empty, or getting to break PPR safely.