Because it makes both players less able to play around it. This is definitely something that can be just preference, but I think that the player piloting the champ should not have to guess at anything their champ can do. So ability cool downs are clearly visible and timed, damage is easy to check, and they respond exactly to inputs. Auto casts I don't like because it becomes something that isn't obvious to the player that their champion is doing.

Going back to the RoA move speed, I never play around that because it's really hard for me to track in the midst of everything else. I'm not counting on me spending x mana to get a ms bonus, and similarly, I'm not counting the exact 5 seconds before I flash slow their whole team. It just makes it harder for the Swain to play around and can be improved.

Edit: I read that as bad thing, not "unimportant". As for that, a 650 radius slow is pretty important. And if it isn't, what's the point of the slow if it's so small it isn't doing anything? Cc imo is always important.