the javelin does an average of 3.5/die on the attack, the longbow 5.5/die.
So with twin strike the javelin does 2 less damage on average (until level 21). not a big difference.
Until you consider that unless the ranger is using the javelin as his primary melee weapon, it's probably way behind on the enchantment curve. My Level 5 ranger just got his +2 double sword, and was able to just barely eek out enough cash around level 4 to pick up a +1 magic javelin for those times when I really do wish I had one. I have a 14 dex and that's not going up except at 11/21 (maybe at 18, I haven't decided whether to go for axes or swords in epic yet). I'm wearing chain but plan to pick up scale by paragon because I want my speed back.
So I've had to sacrifice 2 feats on armor proficiency, just to stay even with the archer ranger (if I continue using double weapons, technically I'm only out 1 because of the defensive property, but I don't want to keep that long-term, I'd lose a lot of power).
I'm not sure it'd be horrible if I was allowed to use twin strike to make ranged attacks... I'd only do it in extreme measures; I'm much better at melee than I ever will be at range, even with using str instead of dex.
And as pointed out, it's really not that hard for archer rangers to be good in melee too, because their AC is as-good, if not better, they can take defensive mobility and/or a beast companion, and if they really miss the HP we get from toughness, they can take that too.
A drow ranger could wield a hand crossbow as a 1d8 high crit weapon and use it from melee without provoking OAs, with the right feats.