I have read the following "general guide lines" about what more exhaust duration is good for:
1) weak flowing exhaust port (less than 70% of intake port)
2) stock exhaust manifolds and/or restrictive exhaust system
3) heavy car with a small engine, like 4500 lb truck with a 350.
4) cars with automatic transmissions
5) N20 or supercharger equipped engines
A cam with more exhaust duration with have a rougher idle than a cam with less exhaust duration as long as the cams are ground with the same lobe separation. The longer duration exhaust cam will have more overlap, and the amount of overlap effects the idle more than anything.