I read the OP and first couple pages, as well as the last couple pages. And yes, I know it's been discussed to death, but that's why I'm in disbelief. I really don't think it's complicated yet here's another 10+ page thread.
Thing is... would it hurt anybody if the engineer would get one "Free Kit Slot"?
Would it hurt engineers? No, those who don't want to use it don't have to.
Would it make the engineer OP? I doubt so, because as people say, you have to switch to it, it frees up one Utility slot, so an engineer isn't infinitely stronger through that change, just more versatile in his utilities when he decides to go kit(s).
Would other classes cry with reason? I doubt so, it's the same arguments every class member throws against every other class: "They can do this better, I can't".
Eles: They get 20 Weapon skills for free, more through Utility Conjures. Engi would then get 10 for free, more through Utility Kits.
Rangers: They get two weapons, utilities, and three Pets which they are fairly free to choose out of... roundabout 36? Each having 4 Extra Skills. Engi would then have two "weapons", utilities, and 4 Toolbelt Skills.
Guardian: Two Weapons, 3(4?) Virtues which they can't choose but which instead always have a passive effect as well, Engi: Two Weapons, 4 Skills to "choose", but no passive effect.
And so on and on. Yes you can't compare classes to one another, but when you try you'll always find all other classes ruling against yours', and all other classes' people find their class sucking against yours'.
Point being: If the engineer had the second slot or not, wouldn't change the complaints or give them more reason.
So the way I see it:
It would make the engineer's builds more versatile, would allow for more builds, would make the engineers' gameplay more interesting while balancing issues, should they occur, can be solved through Number Crunching/Changing.
How many skills you can use doesn't make you stronger or weaker as such, but makes things more interesting IMHO, and that's why I'm afraid that the engineer (especially the Turret Engineer) becomes boring much faster than other classes.
A change which makes engineers more diverse to play while not hurting the balance too much doesn't sound too bad does it? After all, it makes playing AGAINST engineers more diverse/fun as well :)