"Abusing" does not mean simply spamming an ability that can make good use of an item. "Abusing" means exploiting something that is overtuned in order to gain an edge.
Anivia could use it in combination with her ult to add on an even more potent slow. This would make hitting her Q stun easier, making her already massive damage potential even more potent.
Annie's Tibbers consistently deals AoE DoT (damage over time) damage, similar to 
. This burn applies the Rylai's slow, making Tibber's a considerable threat even after being cast.
Azir (as you know) has consistent damage with his soldier's auto attacks, which apply the Rylai's slow.
Brand's passive is a DoT, similar to 
. This burn applies the Rylai's slow, making his skill shots easier to land.
Cass (as you know) has her spammable E, but not only that, the slow would apply off of her Q and W as well, making each of them easier to land as a result.
Heimer's turrets are similar to Azir's soldiers in that they consistently deal damage. They are considered a spell, so they apply the Rylai's slow in order to deal even more damage.
Malz has E, which, like Brand, is a DoT, which consistently applies the Rylai's slow. This makes it easier to land his Q, and makes it easier for his Voidlings to stick to his targets (this is why he being played as a support right now).
Morgana has both her W and her Ult. Her W is a DoT which consistently applies the Rylai's slow, making it easier to land her Q. Her Ult already has a potent slow coupled with it, so Rylai's makes it that much harder to get out of it.
Orianna has a decent slow on her W. Couple this with the Rylai's slow, it makes it easier to land her Q, and Ult.
Syndra's Q is quite spammable. The Rylai's slow would make it easier to land both her W and E as a result.
Swain's Q and E is a DoT, and his ult (while not exactly a DoT) puts out consistent damage. All of these make Rylai's a very important buy on Swain even today, after the nerf.
Taliyah's Q is also quite spammable, and her W has a decent slow. The Rylai's slow would make it easier to land her E, W combo as a result.
Vel'koz is extremely skill shot reliant. The Rylai's slow would make it easier for him to land the rest of his skillshots after landing just one. Also, his ult has a potent slow. The Rylai's slow would make it that much harder to get out of it.
Viktor's ult (altough it isn't a DoT) does put out consistent damage. The Rylai's slow makes it harder to get out of it. Also, Viktor can his Q with Rylais's slow in order to more easily land his E.
Xerath (like Vel'koz) is very skill shot reliant. Landing his Q would make it easier to land both his W and E, and landing one shot of his ult would make it easier to land the rest.
Ziggs is also heavily skill shot reliant. Landing his Q with the Rylai's slow would make it easier to land the rest of his spells as a result.
One champ I actually forgot to add was 
Zyra. Her plants deal consistent damage, which, with the Rylai's slow, makes it easier to land the rest of her abilities.
Yes, some mages abused it more than others (i.e. ). But, like I said, almost every mage could abuse it.
and 
should had not taken that long to nerf since it wasn't really a huge nerf other then to Maokai since obviously he is being played in the LCS.
when rarely any ADC even build these items any more are getting nerf, 
I understand but the rest I don't get.
or 
because:
