People use White Scroll because: The slot they use it on is important. They don't want to lose the slot in any circumstances. People use 30% because: They don't mind losing the slot (which means item destroyed) due to failed scrolling. So why would you combine a White Scroll with a 30%? They are used for different aims.
its a shit idea because since you fail 30% scrolls, 70% of the time, and out of the 70% you boom it 50%. so 35% of the time your item will boom, which means on average you will stand to lose 35% of the value of the item you are scrolling, which costs more than the ws+10%, especially when you consider WHEN it booms, you lose 100% of the claws value at that time. no one should take that risk when you can use a method where the only value you only lose are scrolls.
personally i've always followed the ~1b-3b rule for 30%+ws, depending on how rare the clean equip i'm scrolling is. here's a quote that kinda explains it and here's another person's calculator for when to use 30%+ws that i've used a few times too https://royals.ms/forum/threads/scrolling-calculator-for-perfect-items.127383/
I WS 30%'d the first two slots on both my ele 5 and ele 7. Dumb in retrospect when you're dealing with very rare clean items like those since you only save 5b on average per slot max! I actually can name a few people off the top of my head who WS 30% their weapons. Granted, those people quit and only log on after voting a long time to gach at NLC and use the WSes they get. At least one of them had their weapon boom and then I never saw him again haha. So WS 30% for regular, active players is typically when the clean item is relatively easy to find and isn't too costly. Think 105 att clean nisrocks, 56 att Red Cravens. But a 60 att sleeve? Hell naw.