The SEA cell phone lot is an afterthought, carved out of an area near the cargo / US postal service hangars. Access requires a tricky reverse detour from the main approach road to the terminal. It's often congested, with users parking each other in or idling in the middle of things. It can take ten minutes to fight your way out of there once you get the come-get-me call from the terminal, so, no, it doesn't work very well.

That said, circling the terminal ramp over and over is stupid and contributes to road congestion, and I've seen troopers issuing tickets to motorists who try just pulling over to wait on the shoulder of the approach road.

Protip for SEA: meet your party on the departures level, not the lower arrivals level which is often gridlocked, especially at night.