What are the advantages and disadvantages of elastomeric coatings versus shingles?
If what you are asking is if you should coat old shingles with a elastomeric coat instead of tearing off the old and replace with new, I think the answer is no. Such rubberized compounds are normally used for short term repairs (2 to 5 years) on damaged shingles on a pitched roof, and as a seam sealer on rolled roofing for flatter pitches.
If your shingles are failing, coming loose or have cupped or distorted with age, coating them most likely would be a very short term fix. The underside of the shingles will continue to fail, making an expensive coating project a waste of money in my opinion. 

If I have missed the point of the question, I'm sorry. If the situation is different, the answer may change a bit. But normally, i'd never use an eleastomer coat over old worn out shingles.