Considering the facts that you have mentioned above, Appraisals are essential to know the performers and non-performers. There must be existing Appraisal system in the organization or are you initiating it? I think it is imperative to understand the motive of your employers behind the Appraisals idea.
Even if you are doing the same work, who is 'accountable' to what should be certainly known to your field staff as well as employers.
I am an HR Consultant from Pune. If you are open to services from a Consultant for Appraisal work, please contact me.