https://classic.warcraftlogs.com/reports/RkGTCtAm3fB9XHcd#pins=2%24Off%24%23244F4B%24expression%24ability.Type%20!%3D%20%221%22%20%20AND%20IN%20RANGE%20FROM%20type%20%3D%20%22applydebuff%22%20AND%20ability.name%20%3D%20%22Spell%20Vulnerability%22%20TO%20type%20%3D%20%22removedebuff%22%20AND%20ability.name%20%3D%20%22Spell%20Vulnerability%22%20%20END&type=damage-done&fight=1
https://classic.warcraftlogs.com/reports/VWCKNqP4LBYzXGmD#fight=last&hostility=1&type=auras&source=1&spells=debuffs
https://classic.warcraftlogs.com/reports/ZnWJkjPThD74YQBg#fight=last&type=auras&hostility=1&source=1&spells=debuffs
tl;dr: proc rate in Classic is only 1ppm, instead of 2 as was inferred from old wowhead/thottbot comments.
All that said, again, it’s still probably worth making 1-3 for the long term. We all know casters get absurdly powerful towards the end of vanilla content. It’s just that, as of now, it either breaks even or is inferior to if the shaman or whatever were going for for personal DPS.