What, may I ask, is the point in doing that!?
Same reason as already stated by the OP. To maintain rotation. Pretty simple answer.

"Here, have Fire IV in Sastasha..... but it will do jack all for damage, so you might as well not even use it."
"Here, have [Diamond Armor of Casting] in Sastasha... but it will do jack all for [stats], so you might as well [wear level appropriate gear and support the low level crafting economy."

See how that works. Not exactly mind bending.

And how would you TELL which abilities are good and which aren't? "Gee, is Fire III available in Brayflox normally? I gotta look it up to see if it is even worth casting" .
You're joking right? Open the Actions and Traits menu. Problem solved. Wow! You're really giving me curve balls here.

Really don't see what you'd gain out of making the higher level abilities almost worthless, might as well just simply level Sync. They both do the same exact freaking thing. Well, ok, I suppose you could waste GCDs on worthless abilities.... but why?
"Really don't see what you'd gain out of [wearing level sync'd gear and nerfing it's stats in sync'd dungeons] might as well just simply [level crafting and always have level appropriate gear]. They both do the same exact freaking thing. Well, ok, I suppose you could waste [glamour prisms and gil on emperor's new gear]… but why?"

Wow! You've completely blown my mind! It's like, there's a legit reason why people want to have the convenience to the things they already have unlocked while also showing gear off to their lower level compatriots and friends and encouraging them to stick with the game so they could get those things too. NO WAY MAN WHOA!