I emailed them about the S-tubo being a spare only. This is what I got back, "thanks for approaching us and your interest in our tubes.
Basically you can also use S-Tubo-Road as your daily tube. While Tubo-Road is twice as robust against punctures as standard tubes, S-Tubo-Road is still as robust as standard tubes. To see how we test that you can watch following video:
https://www.youtube.com/watch?v=T5YuWHzVxSI I hope that answer helps.
Please do not hesitate to come back to me with any questions."
So seeing as the S-tubo can be used as an every day tube, and the rolling resistance, particularly at lower pressures, is in line with latex, does that change your opinion? From what I'm seeing, the cost is the only downside.