I listen to KFNQ from time to time. I find sports talk more enjoyable than the political ranting that was on KPTK before it was flipped. Even so, I used to listen to KPTK regularly, up until just before the last election, when they went ballistic with their partisanship. Sometimes I miss hearing them.
KFNQ are providing male sports fans in the Seattle area a third choice. And the three sports networks (four if you include Deportes, which I can't pick up very well) are a bit different, just like the 3 pop stations, 3 country stations (if you include KXA), 3 classic rock / classic hits stations, 2 alternative stations and 2 NPR stations in the Seattle area are a bit different.
As for "compelling", if you're a sports fan (I'm not, I only have a passing interest in sports) you would probably find Scott Ferrell or Jim Rome much more compelling than much of the homogenous music on FM. I don't know how KFNQ's ratings have been but their potential audience (sports fans) is probably much larger than the number of progressive talk fans. Sure, KPTK could fill a hall. But sports fans fill entire stadiums. This is the audience CBS Sports and the other networks are chasing.
And the fact is, Seattle is one of the top 20 markets in the US, and CBS Sports is a national sports network. It would be stupid for them not to want a network outlet here. National networks advertise national coverage.