Pros:

-Reduced labor cost & reduced dump fees which brings the overall project cost down roughly 15% or so depending on the type of roof.

-2 Layers are the maximum number of layers allowed by code in most areas.

Cons:

-You cannot inspect all sheathing to verify a solid nailing surface.

-You also cannot change the underlayment or install ice and water shield / moisture guard to the eaves and valleys direct to plywood.

-Lastly, heat usually builds up between the layers and tends to suffocate the top layer causing pre-mature roof aging.  Ventillation plays a big role in a roof-over application. 

Generally speaking, I would say that roof-over application is not recommended due to cost vs. overall value.....especially if you are planning on staying in the home 10+ years. Hope this helps!

Eric Consuegra, Arocon Roofing and Construction, LLC

Every roof system is different and the length of time that the shingles last depends upon many factors such as ventilation, roof pitch, direct sunlight vs. shade etc.  In Maryland, a 20 year shingle typically lasts 15-17 years due to the extreme heat and extreme cold climate changes.  I would say that if the original shingles on the home were rated for 20 years, chances are it's ready for replacement.  When homeowners push the limits of the roof....they normally end up spending way more money for the replacement due to the plywood damage which results from waiting too long.

Eric, Arocon Roofing & Construction