I'm thinking that the reason the Lv.X are more playable than ex's were is because there are so few of them. When you do stop and think about it, absolutely every single deck that has been even mildly successful all year has had a Lv.X involved. We started with Infernape and Empoleon of course. Next we moved into MT, and then SW, where we picked up the Lucario, Gardevoir, and Magmortar. That right there was the format for a period of time. GE did not necessarily present any new decks by itself, but Cresselia and Palkia both did see some popularity in the decks already in use. MD now has brought us Eeveelutions and Garchomp. It has also, go figure, reinvigorated our first two, Empoleon and Infernape. So from all of that, the decks throughout the year come up looking like this:
Empoleon
Infernape
Mario
Gardellade (picking up Cresselia for some time)
Magmortar
Eeveelutions
Garchomp
Infernape MD
Empoleon MD (picking up Palkia in some variants)
There was also Arithmetic, which utilized Cresselia. Honchkrow was there too, but I really don't remember hearing of much success for it. Oh, and Blissey, but even that had a variant involving Darkrai. Other than that, I can think of no other decks that have gained popularity amongst the general public during the entire year. Can you?
Now someone said that the popular decks did not necessarily require the services of their Lv.X. While that is true, you can't convince me that they are not integral parts of the strategies of their decks. Empoleon likes to use Supreme Command to restrict your opponent's hand while it knocks off a Claydol or other accelerator. Infernape shreds through the deck on Burning Head, and takes an entire game's worth of Fire back into the deck on the attack. Lucario's Stance followed up by a Cessation Crystal was the turning point play in a lot of matches, and it has always been a staple in any Lucario based deck. I know the Bring Down on Gardevoir is an essential part of its game, and something you have to be thinking about during the course of an entire match when you're in a mirror match with it. Flame Bluster wins games with the instant KO of any small pokemon on the opponent's board, and the free burn for the defending pokemon is always nice to have. Leafeon decks have been sprouting up everywhere not as Eeveelutions decks, but solely for Energy Forcing, while Glaceon has been talked about for its power locking nature. And again with the Empoleons from MD, except now they frequently revolve around abusing Palkia with free retreat cost from Mantine.
So you see, the Lv.X have been dominating the format from the start. They may be few at the moment, but they have potentially been keeping any deck without any Lv.X out of playability. That I do not like at all. If they are going to continue getting a result like that, they should scrap the whole thing, or give everyone a Lv.X.