Tends to allow the retention head to get hotter, and the nozzle to over heat. Fouls the nozzle a little quicker.
The spark tends to let the flame be a little further from the head, less heat on the nozzle.
It does it more as the passage ways of the Miller restrict.
Before the Beckett became standard. R8185 Intermitant primaries took out more nozzles then dirty oil did.