I have to go with a very yes no answer here.
Yes it serves a purpose, to all of the people who fall into the: Hardcore Raiding, Legacy bored, and World/Server First groups. Yes because they have had the time to farm the gear that makes lower leveled dungeons trivial and boring. They have had the time to master the instances and are ready for something they can't clear first try every time. Yes because while full collections are cool, being bored and not seeing any real progression sees people leave the game.

No because that(those) group(s) is(are) not the majority of the player base. It may be the majority of the people that post on the forums but we all know that's a very small percentage of who actually plays the game. For the majority adding progressively harder content is fine, so long as it is not to the exclusion of more "challenge appropriate" content. CT is that gap filler (that should have been present already ...) and maybe SE will put 2 and 2 together and realize they dropped the ball with the difficulty tuning on the primals as well. Titan for iLevel 70 but is the hardest primal, Moogle drops ilevel 80 but is easier, Garuda also drops iLevel70 but is easier than moogle ... that's good balancing, but I digress.

In the end, both things should be happening, content for the majority is probably the more financially appropriate choice though, as catering to the majority means keeping more players playing.

Edit: and one day we'll get to post 1000 words instead of 1000 characters ...