Yes, the Amputator taunt is useful. You can use the Medi Gun to heal people individually and get your ÜberCharge up, but you can save a lot of people's lives with the Amputator taunt.
For example, if a Pyro charges at a group that you are healing one-by-one and burns a lot of them, you might not be able to extinguish everyone before they die. The Amputator taunt will help in this scenario by quickly extinguishing all your teammates at once leaving no one behind.
I personally love the Amputator because it helps a lot of people especially when there are a lot of needy people who want you all to themselves. Using the Amputator taunt you can share the love.
You can read more about the taunt here.
Here's a quick comparison of healing methods:
Heals* Healee Duration Charges** Oktoberfest 11 Self 4 s 0 Medicating Melody 25 Team 4.2s 0 (if in sight and range) Medigun 24/s Target - 2.5 %/s Kritzkrieg 24/s Target - 3.13%/s QuickFix (beta) 36/s Target - 3.75%/s * 300% for targets that haven't received damage in the last 3 seconds ** 50% for targets that are at 142.5% health or higher
The Amputator's taunt allows you to heal multiple people at once. Of course, the drawback is that you're standing still and open to attack when you do it.
The Amputator works well on
cp_degrootkeep (Medieval Mode), where you're not allowed to use your medi-guns, and the small size of the map means players are bunched closer together.