I don't think the issue is comparable to silence(not being able to cast), but more so spending more time casting instead of meleeing. the concern i think they have is when it does cast something like a T4/5 spell, or a -ja spell, the base casting time on those are around 8 seconds. If the mob has some native fastcast(most NMs), maybe 5 seconds casting time. With just rdm sabo+addle, that puts it up to 7.5 seconds. if nocturne did 20%, 8.5 seconds total.
Now every time the NM tries to cast those spells, that's an extra 3.5 seconds he cannot do anything. Now, I don't feel it's overpowering, but i can atleast see why they might be cautious. I feel the suggestion of allowing them to stack, but capping total at +50% is a good option.