You might get more dice with more mastery, but they need 4+, while you need 6 (unless the power targets a unit with a psyker). You might slow an opponent down with more mastery, but you probably won't stop them.
Yes, but - the risk of them failing to cast a spell in the first place seems high, and you only need to use your dispell dice if they actually succeed in casting the spell!
Let's say you're playing against an army that has only a lvl 2 librarian. And I have Fateweaver and a level 3 Daemon prince in my army.
His turn. He rolls a 3, so he gets 5 warp charge points, and I get 10.
Let's say he decides to cast a level 2 spell and a level 1 spell.
He's going to need to throw 4 dice at level 2 spell and 1 die at the level 1 spell.
Odds are against both of them going off. The level 1 spell has a 50% chance of failure, level 2 spell has like a 25% chance of failure.
If the level 2 spell gets cast first, and goes off successfully, as the defender I'll just throw all 10 of my dispel dice at it. Odds are about 60-70% that I'll dispel it. And then the librarian still has a 50-50 shot of failing his level one spell after that.
If the level 2 spell fails (not uncommon), now as the defender I have all 10 dispel dice to throw at the remaining spell.
And remember.... we're used to seeing Daemon armies with WAY more dispel dice than this. In 6th it wasn't at all uncommon to see 19 mastery levels on the table rather than the 7 in this example (4 max heralds, fateweaver, daemon prince). That will just shut down anyone who only brings 1 or 2 psykers of their own. It's a new and interesting dynamic for sure.
