I have to agree with you for the most part because I have a flip-flip rear wheel and the only time I setup for SS is when I lending the bike to someone else, and if I don't want the regimen of the fixed gear I would just ride one my geared bikes, and even if I just wanted to to mash up hills and spin down them I would still probably do it on my geared bike. When I ride my geared bikes I normally monitor my speed, my cadence, my perceived workload (no power either), and my heartrate, but when I ride the fixed gear I generally pretty much ignore my cadence and my speed and instead I concentrate on my perceived workload, my heartrate, and my route, because my cadence and my speed are pretty much dictated by the other three.

Instead, when I choose to practice spinning on my fixed gear I choose a fairly flat route and I concentrate my workout on my cadence and my heartrate, and when I am doing intervals on my fixed gear I choose a hillier course and I concentrate my workout on my on a heartrate and power output. To me spinning and interval training are just two completely different types of workouts that require different types of routes when you do them on a fixed gear, but both types of workouts are done quite easily on a fixed gear, but that's just me.....

I don't see the likelihood of getting the same level of workout on a SS vs a FG however, because as you said, coasting pretty much negates the advantage of the FG.