Sure, I realise the math behind it, but we're talking about a game here where psychological elements come into play as much as pure math.
I could buff the ranger, and have a 3 out of 4 chance that he'd still miss anyway and my buff would be wasted, or buff the rogue and only have a 1 in 4 chance it'd be wasted... admittedly from a pure math POV, if the rogue rolls high enough to hit without the buff, then the buff is still wasted, but when a player hits you don't tend to think of a buff as a waste so much as when they miss. At least I don't, and I'd wager most players are similar.
Sure, but that is your choice to do so. It may well be the better tactical choice, if the rogue's damage is significantly higher. (Though I'd be surprised if it was - if the rogue already has a decent chance to hit, then far better to multiply the ranger's chance of hitting
by five, since his damage - even non-optimized - should still be fine.)
But that's the thing, in the end - D&D is a game of choices. In this case, you have one character who has made many choices that would enhance him, and a party that makes the explicit choice to further enhance him. And another character who has avoided any options that would enhance him, and is unsupported by the party.
Up against a monster that I feel, again, probably had unusually high defences. It doesn't sound like a difficult fight, but if an optimized rogue is only hitting on a 13 or 14 by default, its defenses definitely seem high. As you say - apparently intended to challenge optimized characters being enhanced by buffs. Of course it will be untouchable for the non-optimized character not receiving the buffs.
So, with all those factors - yes, there will be a gap. The only way to shorten that gap is to reduce the effect of any individual choice. Reducing the effect of different levels of weapons, reduce the effect of differences in stats, reduce the ability for feats/weapon proficiency/classes to give minor bonuses to hit.
If you go too far with that... characters become identical, and there are
no choices to be made.
There is certainly a line that needs to be set. With the exception of the Expertise feats, I think 4E has done a fantastic job of it, and the problem is
almost nonexistent compared to how it used to be. The Expertise feats have definitely made it worse, though, and I would be glad to see them gone or fixed in some fashion.
But even without that, it wouldn't solve the issue for the character here - someone who has a non-optimized character to start with, who doesn't have even one appropriate magic weapon of his level (cause even dual-wielding, his main-hand should at least be on par with the rest of you guys), who doesn't receive the same support from the party other characters get, and who is fighting encounters specifically geared to challenge an optimized party. His character not being effective in combat is inevitable, and you can't fix that without removing the ability to make the choices that led him there - choices he made, choices the party made, and choices the DM made.