Light Metal is a bad ability because it only reduces the damage done to Scizor for two moves, both of which are rather uncommon, and it also increases the damage done to Scizor by two moves, which are equally uncommon; it's extremely situational and doesn't necessarily give a net gain. And in any case, why would your opponent use Grass Knot or Low Kick on Scizor? It has naturally very high defense, so even without Light Metal, they'll be doing negligible damage, so your opponent is better off using a different move or switching to another Pokémon.
Swarm is also a bad ability because you have to hope your opponent knocks you down to 1/4th health before it activates, and the only worthwhile moves it affects that Technician doesn't are X-Scissor and U-Turn. Granted, those are two very good moves, but it's too unreliable to be worth it.
Technician is always in affect and boosts Bullet Punch, allowing it to seriously threaten a large percentage of Pokémon in its home tier. Here's a standard sweeping Scizor set with Technician against many high-ranked Pokémon in UU:
252+ Atk Technician Scizor Bug Bite vs. 0 HP / 0 Def Hydreigon: 354-416 (108.9 - 128%) -- guaranteed OHKO
252+ Atk Iron Plate Technician Scizor Bullet Punch vs. 0 HP / 0 Def Aerodactyl-Mega: 296-350 (98.3 - 116.2%) -- 87.5% chance to OHKO
252+ Atk Iron Plate Technician Scizor Bullet Punch vs. 92 HP / 0 Def Primarina: 166-196 (51.2 - 60.4%) -- guaranteed 2HKO
And here it is without Technician:
252+ Atk Scizor Bug Bite vs. 0 HP / 0 Def Hydreigon: 236-278 (72.6 - 85.5%) -- guaranteed 2HKO
252+ Atk Iron Plate Scizor Bullet Punch vs. 0 HP / 0 Def Aerodactyl-Mega: 200-236 (66.4 - 78.4%) -- guaranteed 2HKO
252+ Atk Iron Plate Scizor Bullet Punch vs. 92 HP / 0 Def Primarina: 111-132 (34.2 - 40.7%) -- guaranteed 3HKO
Technician often makes it possible to KO opponents a turn sooner, without any need to rely on your opponent to knock you down to 1/4th health without killing you.