Then the same could be argued about raid gear. Until you get a full set, you still have the previous tier's one. Or in case of first tier, dungeon/quest reward/reputation/crafted gear. Moreover, the off pieces you may get can be lookalikes of different spec. In your case, since as far as I'm aware you play a Paladin, you may end with some gear that completes DK/Warrior set instead of paladin one. TBC questing gear was dubbed clown gear because even if you wore quest rewards only, you were hard pressed to find two items that at least had the same colour, let alone matching theme.
And it's not about transmoging during leveling, but after leveling. If the quest set is nice, why the hell not? I used Cata crafted set as my PvP transmog for a very long time.
Also, it's not that leather is easier to design on it's own. It's SAVAGE leather that's easier to design than SAVAGE cloth. Think about it, this whole SAVAGE theme stems from focus on Orc society before the start of Warcraft. A society of hunters and warriors. When they don't wear heavier armor for combat, they chill around in leather and fur clothes. With bones and tusks. Bonus points to SAVAGERY if it's from Draenor's SAVAGE wolves. Makes slightly less sense when you realize there's only one cold area on the continent, but Blizzard doesn't let such details phase them.
So instead of trying to make cloth armor look remotely savage in any way, they just made it leather and fur. On both quest sets and LFRaid set. But as I said, crafted gear looks great. Only the helm is off in my opinion, but there are alternatives. And I saw dungeon sets on WoWHead today, also nice. Also made of cloth.
I'll give you that the second mail set has some scale armor visible on legs, sides and arms beneath all that leather and fur. But it's still tons of leather and fur. LFR set and the first quest set have even worse mail/scale armor to leather/fur ratio. Same goes to leather. The second quest set has more leather, but the first one and LFR set are covered in metal from head to toe. The proportions are off.