I made a detailed vid comparing new Dual Wield A5 merc with A2 merc (for necro). Summary: Act 2 wins because A5 merc has taunt which overrides amp damage and it is disgusting because it is a huge damage loss.
Act 2 does not win because of might. In theory you can stack 2x Last Wish on A5 merc and get level 34 aura (yes 2x last wish does indeed stack and gives level 34 might aura). However A5 merc overriding amp damage is a huge loss. Act 2 merc also has better range since it uses threshers/giant threshers in general. This helps summon necro a lot because your merc can still attack from 2nd and 3rd rows (when the enemy is surrounded by your army).
Speaking for Necromancers, Act 1 Rogue Merc cannot replace A2 merc because ranged attacks cannot effectively apply Crushing Blow. This is a huge dps loss against bosses, not to mention uber bosses.
Act 2 is still the best choice for the reasons highlighted above. Here is my analysis vid comparing A5 vs A2 (for ubers):