Scimitar isn't built because it's way too expensive, and doesn't give nearly as much defense as other defensive items. Why build it when you can get Warmog's for 1000 less gold AND have much more survivability? The strict focus on MR makes it useless if the APC isn't getting fed AND it has no health, which is much better, especially in the health stacking meta and on a character who already has naturally low HP (hence why GA has fallen out of favor for Warmog's and FM, despite its great active) -- the stats on it aren't great aside from the active, which has too long of a cooldown to make it worth the cost for the active alone. Not to mention that 60AD is isn't much by the time you're ready to get your defensive item.

Needs a rework in a big way. I reckon they should make QSS build out of a null-magic instead of a Negatron, add cloth armor, and bump the AD up to 65 or even 70. It would make it a more viable option as a sort of defensive-offensive item hybrid that you could get on top of a Warmog's or a FM etc. Works well in a niche area, and still allows you to build other defensive items. Alternatively, to keep it a strictly defensive pick, just add some health to it and reduce the combine cost by 200-400g. It will NEVER become a defensive ADC item without health, and is totally useless right now.

It's too expensive and the stats aren't worth it -- the only thing good about it is the active, which isn't worth 3700 gold.

Yes, the active is supposed to be the major thing about it. You are not really supposed to build mercurial scimitar in every case, it is mroe of a "If I need to build QSS because I absolutely require the active, and then have an abundance of money and a full build already, MIGHT as well build it into MS"

For adc It would be more viable if they increased the damage or the survivability. Nearing your final build, when adc is going to get a defensive item it's better to just get a warmogs or a frozen mallet for the straight health. MS needs more damage to be an offensive item, or more defensive stats to be a defensive item. It's a fairly niche item atm. Only when the other team has LOTS of AP and CC.

Previous Options to Deal with Supression or Heavy CC

Quicksilver Sash

New Options to Deal with Suppression of Heavy CC

Quicksilver Sash -> Mercurial Scimitar

I don't see the problem here.

Quicksilver Sash (1550g)
Negatron Cloak (720g)
830g
B. F. Sword (1550g)
3700g (600g)

i agree
for 830 g + 600 g as total 1430g
they could add more items
like null mantle and a kind of ad item
and as someone mentioned
30% cc reduction for 5 or 3 seconds would be a good choice

and this is really stupid passive
Melee champions also gain +50% movement speed for 1 second
what can i do for 1 second even with master yi?

90s cooldown why isnt it 60s?
its overpriced
the price would stay as it is but the items should become stronger
as i would always start with malmortius instead scimitar
malmortius + frozen mallet = 1100 mgc shield

If the unique didn't have such a long cooldown timer I could see realistically picking the item up. I mean, in theory one could cleanse fear, 5 stacks of bleed, and an amumu ultimate at the same time. In practice, it removes a Teemo blind or you evaporate 0.1 seconds after using it in the above situation.

Um, items with mixed attributes are considerably better than ones with a singular attribute. There's a reason Bruisers win 1v1s. That's actually pretty much the reason bruisers win 1v1s. If you can both survive a boss-mode burst combo and kill a tough opponent in a reasonable time frame you tend to kill a tough opponent in a reasonable time frame. If you cause a boss-mode burst to bounce off you but hit like a kitten, you're target is going to get away, and/or get help. If you can't survive an epic burst combo - you evaporate instantly when bursted.

Ever fight an Akali as an AD champion? It's pretty broken the itemization you have for that - you can go Maw of Malmortis, Wit's End, Vampiric Scepter, and Mercury Treads - allowing you to both outlast whatever damage she deals and deal tons of damage yourself without really straying that far off from a normal build. The reason Atma's and Wriggle's Lantern are pretty much it for offering AD and Armor is because doing the same thing against AD champions would be infinity better than grabbing a mix, making such a build core on basically every top-lane champion. There's nothing quite like being able to pick up EHP and DPS without having to choose one or the other.

As for grabbing it on AD Carries - the AD is a horrible stat on it, really. I'm already forced to get a lot of AD if I want critical strike damage, armor penetration, and life steal. And AD Carries are AD Carries due to the capacity to cause their damage output to grow exponentially through multiplying stats off of each other, not stacking the same AD stat over and over again. At least if it offered something like Critical Strike Chance it would seem interesting - and be an alternative to Hexdrinker/Maw of Malmortis.