Of course nobody is going to center a deck around it.
But it does help against some stuff.
It helps against:
-Sceptile GE (no double energy)
-Bronzong MD (no inbetween dmg)
-Dusknoir SF17 (see above)
-Abomasnow SF (no bench/active protection)
-Aerodactyl MD (no pokepowaa dmg)
-Ampharos SW(no supporter dmg)
-Azelf lv.X (no removal of weakness)
-Cherrim SF (no extra damage)
-Darkrai lv.X (no extra damage)
-Espeon/Umbreon GE (no extra hp or free retreat)
-Garchomp MT (no extra 40 damage)
-Glaceon lv.X (no power denial)
-Gyarados MT (no using magikarp rage)
-Heatran lv.X (no auto-tails on burn)
-Luxray (no extra damage if lv.X in play)
-Machamp lv.X (no extra 60 damage)
-Magmortar SW (no 20 healing on energy attach)
-Magnezone DP (no free retreat w/ metal energy)
-Mewtwo lv.X (no immunity to basics)
-Mr. Mime (no 2 energy or less wall)
-Raichu lv.X (no extra attack - yes it's a poke-body)
-Regigigas LA (no status heal on attachment)
-Regigigas LA (no slow start - GOOD THING)
-Scizor SF (no protection if 60 or more dmg on it)
-Tangrowth SF (no inbetween heal)
-Tyranitar SF (no recycle energy from discard on pokepowaa use)
And some other cards that aren't as popular.
There is a lot actually hurt by Dialga G lv.X, and I think it will see some play because of it.
Thanks Prime for taking the time to make that list. The question now is which of these are still going to be playable. Here are my lists:
Not going to be played as much:
Sceptile,Tyranitar: If your not going to have the energy acceleration then why bother?
Bronzong, Dusknoir, Aerodactyl, Ampharos: Again why waste deck space and in game resources
to get something out if it's not going to be doing it's job. Bronzong is the only one that can't be played around, so the other three have that going against them as well.
Machamp Lv. X, Cherrim, Darkrai Lv. X, Garchomp, Luxray, Gyarados: It's like Sceptile if your not going to get the bonus why bother? Machamp could get the worst from all these, with the trainer that picks up G Pokemon, your opponent could use your Machamp Lv. X against you.
Mr. Mime, Mewtwo Lv. X: No longer a good tech.
Espeon/Umbreon: No bonus, no play.
Magnezone: It's free retreat, IMO, was overrated. But I could see why people still liked it, as it was the focus of many Magnezone decks with Palkia Lv. X.
Tangrowth: It's body is as annoying as an Abomasnow swarm. But Tangrowth had a few buddies crippled by Dialga, Cherrim and the more important one Sceptile.
These could do fine without their Bodies:
Abomasnow: Still a great spreader, it even hits the Dialga, so with time or some techs to take it out faster like Crobat G, Cresslia Lv. X, Spiritomb, Blower (to either bring it up, or go on the flip to put damage on it faster) the Dialga could be KOed in a few turns.
Regigigas (status healer): Not really needed NOW, but we'll see after the next set comes out. Unown G prevents the status conditions from attacks so it can still get around some.
Scizor: It still could be a great T2 deck but the Cherrim, which was it's greatest combo, is the one that is hurt by it most.
Raichu Lv. X: It could get still get to used it's body if the Dialga player is having trouble getting out the Palkia. Heck it could even spend 2 turns doing 80 to it to KO it. In that time Raichu would most likely get KOed by your opponent's main attacker, but that's fine because you could Premier Ball the Raichu and this time actually use the body.
Heatran: It's back to a flip, oh well. The power is the better of the 2 benefits Heatran gives fire decks IMO.
Azelf Lv. X: It's still a great sniper, it could even snipe the Dialga in a spread deck. It's body is good but it's attack is still nasty.
Magmortar: It's body is nice, but still has two great attacks, three if you include the Lv. X. If the body is really needed in a match-up, it could even hit the Dialga for 20 with it's 40/20/20 attack and then snipe it with the Magmortar Lv. X to KO the Dialga. But I see Blastcatty being a really good deck, and one that would tech in Dialga. So Magmortar may have seen it's last few plays in these CCs unless something comes out in the next set that could help it with it's Water match-ups.
Now with all that being said there will still be ways around Dialga G Lv. X, like Poke+ Blower. A lot more people could start playing these to "gust" up the Dialga and hit it as the active. Or even spread decks, could bring it up and snipe around it until your opponent powers it up to retreat or has another way of getting it out of the active position. Some might still work, T-Tar could still be powered up by Weavile or Delcatty/Togekiss and run their own Dialga to counter other bodies that many hurt it. It will all depends on how much the format/Metagame is affected once the set is released. Right now we could all speculate as too what can be good from reading spoilers but we won't really get down to really thinking about combos, techs, and counters until we see how the cards are translated and ruled once released. It's also much easier to think when you have an image to look at then just reading some text on the internet. At least I know it helps me more, because I memorize what almost every card does and then break done how they interact with each other.
No tech is perfect. It's like saying you shouldn't play Uxie because it could be sniped or Pokeblowered.