Are the grottos the only benefit of active membership?
No. The grotto system is a very minor part of active membership, and in fact is being severely modified and phased out.
when one agrees to becoming a media representative what should one expect?
To fulfill the duties of an Agent should they be offered to you, and to represent the Church positively at all times. Whether or not such is offered to you will depend on your ability to demonstrate yourself as capable of representing the Church.
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."