So it’s my understanding, your question is: “clear list button is useful or not useful”
Let me ask you a question first. Why did you think that your product need it? did you observed that user leaves your product because it’s hard to delete all? or it’s just your personal thought that user will need it?
useful or not useful, that’s the question!
IMHO, This is the one of the main question we need to find the answer as a UXer.
To determine something useful, users try to use it.
If they can use it "efficiently":
not useful (useless)
Of course determining usefulness is much more complex process than this. I just want to show the main idea.
We need users who use the product
If users didn’t need it to use it, we wouldn’t talk about UX domain or at least Usability. We would handle things by asking the boss and co-workers.
We should observe the truth, not ask directly
Another important thing that’s my understanding of UX is : there’s a difference between finding answer and getting answer.
Getting answer is simple. you ask question directly. Do you like this color or not? Do you think this button useful or not?
Answers user give us doesn’t show the truth as always, which’s the reason. To find real answer, we shouldn’t ask direct question the user, instead observe it.
If you consider us as a user, instead of asking. It’s usefull or not? You can tell us:
“You’re a user who wants to clear all product from list and tell us how would you do it”
At this point we need a protoype to test it and lastly we should keep in mind not to forget that we (forum users) might not represent real users.