Okay, yeah, I slightly mis-remembered.
Although.. how many people noticed the pre-Reactor parts of Cortana were based off the internal design of a Carrier in the final Halo 3? The skull and Cortana terminal are in a room that has the same base geometry as Halfjaw's ship bridge (if you take away the flood infestation)
A significant number of those people only showed up for the weekend playlist anyway, and mostly just took population off of Team Slayer and Social Slayer. If you made them weekend lists, then those people would just show up for the weekend.
I also imagine rigging playlists to turn themselves off and on for the weekend was a headache, because if it were easy, Halo 3 would still be having Double EXPs rotating.