Very good question, many will agree it's not necessary to be present during the appraisal, however, i have a different opinion on that. It's a great opportunity to get another look at the property, a chance to catch something that may have been missed or get a different perspective on something.
My feeling is it's YOUR money that's paying for the service and also the property so you should be involved. Understand what your paying for and it's also a chance to possibly learn a few things along the way.
In the past, there were too many assumptions being made, and course realtors won't prefer the buyers to be on site, it's another risk they catch something that may potentially halt the deal, which means they lose their commission.
So my opinion, stay involved, too many people allowed for too many assumptions which led to this mortgage debacle we're going through. Good Luck!