First, neither firehusky nor thesecondbest are right on Scizor. I'm going to be adressing the SpDef/Defensive Scizor with a bit if not no attack investment since it is quite common as a pivot and setup sweeper. I would not want to see my Scizor Knock off, take 30% from recoil then get dragon tailed out, seeing as that damage, coupled with hazards would mean my Scizor can't use its bulk as a switch in. To be honest this argument is both trivial and very situational. It doesn't really show anything on the viability of Garchomp or its ability to be effective. Quite simply, Garchomp annoys physical attackers and gets up hazards easily and effectively (to an extent).
Next, I can adress usage and viability. Is there a direct correlation between usage and viability? Yes but to an extent. Usage determines what pokemon are good in the current metagame and which ones aren't. How good and how bad? Not quite. The top 15 from May was:
Heatran, Garchomp, Clefable, Keldeo, Lando T, Bisharp, Skarmory, Ferrothorn, Rotom-W, Latias, Excadrill, Latios, Talonflame, Chansey.
Now if you are going to compare Scizor with Gliscor and put them on the same level, why is Scizor at 9, while Gliscor at 31? We have viability discussions because usage stats are not enough to seperate the best of the best.
Now I can talk about Scizor from A+ to A.
Here are a couple sets Mega Scizor commonly uses:
Bulky SD (SD, Roost, BP, Bug Bite/Knock Off/U-turn)
Offensive SD (SD, BP, 2 of Bug Bite/Knock Off/Superpower)
Defog (Roost, Defog, U-turn, BP)
Baton Pass (SD/Agility, Baton Pass, 2 good moves)
Quickly touching upon Baton Pass, it is a nasty surprise for unsuspecting players. Most people, including me, would think Bulky/Offensive when I see SD. Baton Pass is a surprise factor and can be very effective when played like it. In terms of viablity, it isn't as good as other sets, but it isn't bad either.
Defog Scizor is a good set to. Scizor can switch in and threaten a lot of the metagame. Mega Diancie, Kyurem B, Clefable, Bisharp, Lando T, Tyranitar, Weavile, and Tornadus lacking Heat Wave. We can even go and stretch this to Alakazam, Metagross, Gliscor, Mega Aerodactyl, Mega Altaria, Clefable and Hippowdon. That's a lot of common pokemon in the metagame. In fact all of these are in the top 50 of usage (except for Aerodactyl) and are A- or higher. The switch in opportunities it has to Defog or get momentum are perfect examples of why it is a good pokemon.
With Offensive SD, Scizor loses its versatility defensively and trades it for an easier time sweeping. Would be switch ins like Heatran and Slowbro risk getting hit with moves like Superpower and Bug Bite respectively. Scizor isn't the best offensive sweeper in the tier, but for fatter and slower teams it can be extremely effective. Additionally, its natural 70/140/100 helps it take a hit and return with a +2 attack vs offense while threatening with Bullet Punch.
Last, its Bulky SD shares qualities with both the Offensive SD set and the Defog set. Combining numerous switch in opportunities and threats of sweeping your team, it's by far Scizor's best set, and additionally is a great pivot.
Now first lets compare
to its fellow A+ ranked pokemon:
notice that these pokemon are all extremely versatile. I don't neccessarily mean the sets that they can run, but the moves they can use in conjunction with an ability to fulfill a role extremely well. Does Scizor possess these qualities? Yes. All of these pokemon have defensive versatility. At the same time, it feels like Scizor doesn't do its job nearly as effectively as the other pokemon in this list.
Next we can compare with the A ranked pokemon
These pokemon are much more predictable than the ones in A+
Ttar, Excadrill, Latias, Gliscor, Skarmory, and Rotom-W feature the most versatility. Pokemon like Charizard Y, Slowbro, Venusaur, Mega Gardevoir and Manectric are rather one dimensional. I'm aware Alakazam and Charizard Y to name a few, can have different surprises like Encore/Taunt or Earthquake/phsyical set for Zam and Zard respectively. But the predictability of these pokemon is much more than those in A+
Also let me quickly mention the Scald argument and how it causes burns. Every physical attacker except for thsoe immune to burns suffer from the risk of burns, that's what all physical attackers in this metagame deal with.
To me, Scizor simply doesn't matchup in A+, which is why I think it should drop into A, where it will be one of the best in that rank.
I support Mega Scizor from A+ to A