For example, on a CFM56, it is detrimental to select ignition in almost any case according to CFM.... reason being it can detect a flame out more quickly than each spark on continuous; and if the spark is required due flameout there is a higher voltage available thus a stronger spark available in a fraction of a second due to not continually discharching the capacitors in the ignitor box. At the airline I work at it is SOP to select ignition on the CFMs in Emer descent... no idea why.