You're missing some points.
1. You can still take defensive mobility.
You can still take Toughness.
2. There are items, e.g. Point Blank Weapon, which provide the same benefit, saving you a feat.
You can take an item that gives you bonus hit points.
3. How often does a ranger actually take attacks of opportunity for making ranged attacks in the first place? Shifting will save you 80% of the time, shifting + nimble strike will save you another 10% of the time, and the ranger has plenty of attack or utility powers which let you shift more squares (e.g. Serpentine Dodge or the 1st level encounter attack power Evasive Strike) will save you a further 9% of the time.
If you're taking a lot of opportunity attacks in the first, you're doing something wrong.
That's true, if you're taking a -lot-. But you don't need to take a lot for Defensive Mobility to be useful- You only need to take -one-.
But the same argument in reverse. The two-weapon-fighting bonus is an -excellent- advantage, should you find yourself in a situation with two weapons and in each hand. But the archery ranger doesn't find himself in -that- situation either.
And not all archers take shift-and-pew-pew powers (tho they are good options to be sure.) For those that take the non-shifty big-heavy-damage-PEW-PEW powers, Defensive Mobility is still better than Toughness.
As well, DM opens up the option of drawing OAs so your fighter gets a free CC attack now and then. Toughness opens up the option of taking five more damage. Which, again, drop in the bucket.
I don't think the situation is as 'obvious' as people think it is. 5 hitpoints is an extra what to your surge value? 1 point? 2 at best? Sure it gets better at higher tiers, but so much that it becomes must have for a ranged character?
If you're going ranged, your defenses are of the DO NOT HIT ME variety, not the HIT ME LESS HARD variety. Avoidance, rather then Mitigation. And Defensive Mobility increases your Avoidance while you run-and-gun, while Mitigation does not protect you against your bane.
Toughness does NOTHING against Soldier monsters, which are the weakness of a ranged Striker. Defensive Mobility does. And that's the -real- bottom line.
In pure comparison, yes both builds get Prime Shot, but archery rangers will actually use it, whereas twf rangers will not. In converse, twf rangers NEED the two-weapon advantage to be competitive with archery rangers in the damage area, but archery rangers don't need to TWF in order to do damage; TWF+Superior weapon=Archery Ranger+martial weapon.
You -can't- look at it as strict 'So and So gets X class features and So and So gets Y so X > Y' when you look at the sum of the parts.
Bottom line, if you take TWF, you're taking an extra feat to be equivalent to archery rangers in the damage catagory, either by taking Defensive Mobility to be as mobile as an archer just to get that 'useful' two-weapon fighting advantage, or you're taking a Superior Weapon feat just to get the same damage output.
It is -definately- not a wash when you look at all parts put together.