As for the Magnetron: it's a support + siege unit, it's not going to have bigger range because it will make its support power overtuned. It's not a dedicated anti-structure unit, so there is really no point in touching it. And its armor is the same as most of the siege units. Plus, repair units exist, you can tank out the damage of T3 defenses, which deal relatively low damage to Magnetrons to say.
If the sieging mechanics of a Magnetron is secondary, it still at least needs even a minimal buff against defenses. It's basically the only effective Anti-structure unit PsiCorps have. Libra does deal decent damage against large structures, but she deals low damage against defenses. While using stingers w/ engies are indeed great, it would still cost you, because you won't just obviously build one each (IME, the minimal was 4 to ensure non of my magnetrons was destroyed).
In PvP, the enemy won't just let you destroy their defenses. It's no doubt they'll retaliate and will try to attack your forces with what they have. That's what the extra stingers are too. While you can say "Why not mind-control the enemies' forces to destroy that defense?" It won't be that easy as you would probably use mind-controlled units to hold your ground rather than forcing them to attack a defense, unless it's a siege unit too. Let's be real tho, no idiot will put a siege unit near a mastermind/elite unless it's intentional.