At most sights, when you use the card you are in the same line as those who made a timed reservation. Also, most who buy the card will not visit enough sights to "break even" on the cost of the card.
That being said, it doesn't mean that there is no value to getting a a Firenze card. The best thing about the card for me is that is gives you complete flexibility. You don't have to be anywhere at any given time but you still have access the same as if you'd made a reservation. The second advantage is that, if a particular sight didn't take you as long as planned, you may may stop somewhere that you didn't originally plan because it is covered by the card.
On our visit to Florence we bought the card. We visited the Uffizi, Academia, the Bargello, the Duomo sights and the Pitti Palace. That's about 53 euros in admission fees. We also saw the Medici Chapel, because we were walking right past it and it was covered, and the Gallileo musuem, really interesting and a welcome relief from the art overload that you can experience in Florence. That's another 17 euros. Across three days, that's not really sightseeing at a breakneck pace and we effectively "lost" two euros on each of our cards.
We woke up the morning we planned to go to the Uffizi and it was a wonderful sunny day. We were able to change our plan and visit the Pitti place instead, enjoyng outdoor spaces on a beautiful day and went to the Uffizi the following day. A timed reservation wouldn't have allowed us to do that. For me, that kind of flexibility has value. You need to decide if it has the same value for you.