It is a good place to monitor the outside temperature as it is unaffected by sun direction.
I have Temp and RH sensors in @joe90's MVHR, but he has not balanced it yet, and turns it off sometimes.
I think that 'must' needs to be in bold, as in always install one the correct size.
I think it is a problem that came about with traditional 'plumbers'. They just do not understand how a HP works. I would think that a buffer is useful in most heating systems to help efficiency.