Things change and it's 2012
There are different elastomeric products on the market that can be sprayed on or rolled on a shingle roof. Some of them "breathe" and some of them don't. The products that breathe will eventually leak under the weight of pooled water. The guys that make products that don't breath sometime claim their products are superior. Ain't necessarily so. Their products just may be superior on flat roofs.
If you are coating a shingle roof I would use an elastomeric product that breathes. Shingles breathe when installed on a slope roof. When you apply a elastomeric product to a shingle roof you are welding the shingles together. The advantage to that is elastomeric treated roofs can withstand hurricane level winds. But if the elastomeric roof coating isn't microporous (i.e. doesn't breathe) you'd better hope the roof has excellent ventilation. We all know most don't.
Techniseal (TM) makes a product that is microporous, as do others. Techniseal makes their product in several colours as well as clearcoat. This company has been selling a shingle coating product for 15 years.
LiquidRubber (TM) makes products that don't breathe. Excellent for flat roofs; the British Museam used their product.
As for guys who say, "Why bother?, just strip and reshingle", I gotta ask you, why would you spend twice as much money (or more) to put on new shingles? My last so called "20 year" shingles lasted 8 years. I didn't want to repeat that experience. That's why I started researching alternatives. And don't tell me it's my fault for having poor ventilation. I repeatedly asked the roofer if I needed more vents and was told no. Of course, maybe that just says something about roofers (BTW, the company we hired had been in business for 11 years, hardly a fly by night).
My opinion now is the only reason to shingle is because you plan on selling your house in the next couple of years and the roof is too far gone to apply an elastomeric coating to it. If you plan to stay in your house and need a roof, apply an elastomeric coating or get a metal roof (or other alternative to shingles).
Shingles are as dead as $60 a barrel oil. Which, come to think of it, is why they aren't worth using anymore. At $100 a barrel there's hardly any oil in an asphalt shingle anymore. And so they just don't last.