What's needed more for modern working patterns is a 'no more than three / four return trips per seven day week' season, irrespective on what days of the week those trips are used on. The main problem with that is a quick and effective way to enforce that condition to prevent over-use. The best...
It does apply to peak time fares, provided they are over the minimum fare of £12 (or £18.60 for London zone 1-6 travelcard) and they are for individual tickets rather than seasons.