As for the thief, it is like 9,000x less build flexible and interesting than the rogue.
Out of combat the thief has actively more build flexibility than the rogue by covering argument (it has access to all the rogue utility powers plus a few things the rogue doesn't, like an extra trained skill and acrobat's trick and sneak's trick (and arguably escape artist's trick) both being utility powers with non-combat uses that don't require attacking).
Build flexible for combat I'll grant. But I don't care about build-flexible at the table; I care about the character I am playing at the table. The thief only has a limited number of builds, granted. But they are more divergent from other 4e classes than anything the rogue has to offer, and they work well or they obviously don't work.
Also there are off-stat builds for the thief that are slightly sub-optimal but definitely playable which wouldn't work at all for the PHB rogue; none of the thief's powers are hard coded to dexterity so you don't have to make it your primary stat as long as you basic attack with something else (normally Str but Int is viable and Cha is possible with a loss of damage). So outside combat a thief is significantly more rather than less build-flexible than a PHB rogue.
As for interesting, I find in play seeing whether I can pull off a death from above via acrobat's trick and still get sneak attack or elbow a minion in the head and slice up my main target with tumbling trick more interesting than just setting up a blinding barrage or using a dazing strike again (to pick two of the more ubiquitous stand-and-hit powers)
I'm not discounting the rogue's strengths over the thief here but the advantages are far from all one sided.
Nor do I think, personally, that the rogue is your light skirmisher. In monster role parlance you have skirmisher, lurker, and soldier. These are the ranger, the rogue, and the fighter, with the warlord taking on the leader role.
Nope. In monster role parlance you have soldier (fighter), skirmisher (rogue), and artillery (archer-ranger). Some skirmishers even get Sneak Attack.
The closest to a lurker you get on the player side is the assassin; lurkers are built round disappearing or otherwise going semi-invulnerable for one round in two.
And I break down skirmishers into two types; one of them is very vanilla and always does medium damage, melee or ranged. The other has low but situational high damage with a mechanic like Sneak Attack or the kobold quickblade's extra damage for each square shifted.
But however you cut it the rogue is a textbook skirmisher and the dex-ranger is textbook artillery. The str-ranger is probably also a skirmisher. But having only the less popular and less well implemented half of the class mapping to a role makes it less of an example of one than being the textbook example.
And I'll go a step further and say that the two weapon ranger in the PHB was a mistake. Not only is it short of powers (v-shaped) but it goes down way too easily because the actual defensive tech is poor, the AC only scales off Dex, and they are aggro magnets because they are glass cannons with no real ranged fallback.
