Soul cap is weird, I hate it and love it at the same time. Theres no better feeling than having Spirit Shift trigger as all my cooldowns are up, but there's no worse feeling than having to cancel it's proc for a priority add. I would never use Soul Cap for Mythic Gorefiend progression and we seemed to push phases much quicker on Tyrant Velhari when all of our rogues switched Soul Cap with Censor. In theory it's a cool trinket, but like you said it's just not very practical for progression raiding.