Dungeons are supposed to be a stepping stone, a training grounds, basic gearing up and basic challenges, challenging enough to help you teach the basics of your class and the game. Something that I do not really think WoW had since the ZA/ZG instances.
If you go further into the expansion, you did not get any new dungeons when ulduar released, so dungeons did not get you raid ready at all here, you had to go through old raids.
When ToC released you got one 5 player dungeon, before FL you had ZA/ZG in cata. The difference here is that you could do ToC5HC one time a day and ZA/ZG you could spam with random queue, so you could gear up a lot faste rin ZA/ZG than in ToC5HC, same story in ICC with the ICC dungeons vs Cata End Time dungeons, you could only do the WotLK ones once, you could spam the Cata ones, making you gear up faster. In WotLK you also only got emblems once a day, in Cata you could spam justice points for easy gear up, there was no "soft cap" with justice as there was on emblems.
So really, Cata dungeons offered much better gearing up than WotLK dungeons
Now in MoP, it is exactly the same thing. I dinged a new tank alt in 5.2. After 2 days it was 485 ilvl, did entire ToT LFR, Nalak, Oondasta, Galleon and Sha of Fear and got 0 items, so was with very bad luck. With some luck it could easily have been 495-500 ilvl after 2 days. Easily raid ready for ToT after just a day or two.