You bring up the point about not 'being able' to send a heavy to go murder instead of holding. So hold with a solo. Go kill that heavy so it never gets the chance to contents in the first place. I've never used anything bigger than a cheap unit to contest / control when it could instead be killing, typically it's some death stalker standing in the zone or near the objective not my Carni. At times the Ravagore takes that, since he does his job back there anyway. If you're holding with your avatar, you're likely losing that game because you're pulling so many points out of the fight to sit and do nothing. You still move them with the scenario in mind, gazing at the right spot, killing what could get around you, or what could threaten that avatar if able. But you're arguing a pretense that shouldn't be happening hardly ever unless that's part of the plan (i.e centurions love to stand in that zone and shout 'bring it'), but that just shows that the game is balanced with more than one way to win, and don'st need a poorly thought out radical change to the game that can only be done with an entirely new edition to the game. There's less logic there than the constant bickering to re-balance focus and fury.
Your other point is on equally bad ground, that it doesn't always make 'fluff sense' for a given caster to be participating in a Scenario is just wrong. Flat out. I challenge you to give me an SR2012 match up with any two casters in the game and any list that I could not write at least a paragraph, if not a short story, about.