I am a level 47 SCH and now am working on my THM to get Swiftcast. It will be an extremely useful addition, but up until this point it has not been necessary, but would have been incredibly useful in many circumstance. As for stoneskin I am still waiting to hear the word on this one. For SCH Stoneskin is gimped from 18% down to just 10%. That's a pretty significant hit.
You will want the additional buffer to use, stack-able with Adloquium, scalable on your ally's hp. If I remember correctly. Adloquium falls first, giving you the ability to rotate shields when wanted, and stack additionally upon a vulnerable ally in anticipation of them being targeted for a heavy damage skill.
Effectively stack it on your tank, and it will buy you time to spread dots in most small encounters, allowing you to shorten fights for the run while keeping your allies safe and alive.
Regardless, 10% of 3k HP is 300 health. Given all the other shields and damage reductions we are capable of providing, its a nice additional buffer and well worth getting. If you're not making use of swiftcast often, this is your go-to for it to refresh on a tank in a tight spot.