Maybe I'm misunderstanding you, but they definitely use 3 buckets in this cabin. It just happens that they don't release a lot of the cheapest bucket for nonstop itineraries. Married segments gives them a lot more control over managing yields. They can also do this with published fares -- i.e., they can have I or even D fares with various restrictions (e.g., only Tues/Wed/Sat, only after 8pm, excluding these specific dates, even round-trip/minimum stay requirements) to further segment pricing.