Perma-slow < Power slow
This is partially an Ashe topic, but it's also a thread on how the concept of perma-slows in a Champion's kit are basically... bad design compared to a Champion with a powerful, limited-duration slow.
With perma-slows, you can kite slow, low-mobility opponents, but once a gap-closer jumps on you, you can't kite or orb-walk them because they're too close to do so, or will keep jumping on you. Additionally, very few Champions in the roster lack a gap-closer and/or some means of retaliating at range. However, there is no timing window for when you can engage and not get slowed; you will ALWAYS get slowed, even if it's relatively weak.
With power slows, an enemy could jump onto you, but then you could slow them and dash/flash away and pepper them to death while they can't move. Additionally, power slows generally are stronger than perma-slows, because they require timing/accuracy/etc. Permaslows generally are too easy to apply and, because they're constant, must be kept on the weak side. (Case in point: Ashe's Q)
In Ashe's case, I would even argue that her permaslow actually hurts her now. She has to constantly shoot to keep applying it, but that means constantly stopping and compromising her positioning, letting enemies keep up with her too easily. This is instead of someone like Jinx, who could Zap! someone and then run back for a second or two before re-engaging. Ashe can't quite do that because her slow is less impactful. Only thing Ashe really has going for her in that regard is her ultimate, which isn't effective at close-range.
If Ashe were made today, I would bet that Riot would not have put a perma-slow toggle on her, but instead would've given her a limited duration steroid that temporarily gave her slowing attacks. Why? They could have a stronger slow, but would be counterable by timing windows; engage when Ashe's slow is on cooldown and she's screwed. Better gameplay.
Riot has also stated in the past that they hate Frozen Mallet's design because once you hit someone with it, they are unable to kite you and must stand and fight, destroying some of their counterplay with little ability to counter said counter (except to not get caught in the first place, which is bogus). And while some Champions can quasi-perma slow you, that generally requires them either to use good ability chains, or requires them to have good teamplay.
In essence, I feel that actual perma-slowing (on one Champion's kit, in particular) in League should be removed. It doesn't make for a good gimmick anymore, and really hurts the one Champion who sports it as her gimmick: Ashe. Riot should look into altering her perma-slow into a power slow, and maybe then she'll see some room for improvements.