Reply to "it is good"

Thank you very much, David,

How about these sentences:

5) It is good staying here.

6) It is useless staying here.

7) It is not much use staying here.

Would you say they need a comma?

I don't know why '1' and '3' seem very bad to me and these three don't seem that bad.

I assure you that when I am speaking or writing I try to stay clear of this structure altogether and use the infinitive, just as you suggested.