Jock,

Simplicity of design wouldn't factor that much, really. It would depend how often a certain element is repeated within the sett. If only once, then you would have to use the whole sett per pleat, regardless of whether you're pleating to the sett or stripe. If an element occurs more than once, then more pleats per sett could be produced.

Kilts made in really big setts tend to have fewer pleats overall than their smaller sett counterparts.