Dialga is a deadweight. It is just a mediocre attacker unless you load it with 2-3 special metals. So now you're talking about adding a 1-1 Dialga, 1 basic metal and at least 3 special metal. That's at least 6 cards to only have another trick against Machamp (which probably doesn't change the 5050 matchup status), to beat random stuff like Ampharos which didn't appear, and to have another toy vs mirror which I don't think helps much at all. Palkia can be offensive getting draw+Mesprit and keeping the azelf lock and it also breaks the opponent's azelf lock while Dialga can only break the opposing lock.
I played only a couple games of trying a DPL list but it only reconfirmed that Dialga was just a usually unnecessary third line that took up 6-7 cards! It doesn't nearly warrant that many cards. I hated some of the things you told me you did to your list, Chris. Mainly the 1 Power Spray. You have 2 greatly disruptive cards in Luxray and Palkia, and a not disruptive one in Dialga.
As for just a 1-1 Dialga and 1 basic Metal over 1-1 Mismagius and 1 Bebe, with no sp metal I think the Dialga would only get used vs Mewtwo. Maybe I'd find a great situation to use deafen or something, but I'd be afraid of Flygon/Mewtwos finding a way to keep the Mewtwo alive and snipe the Dialga x. There are no energy I could drop from this list for special metals. 4 Water 2 Lightning 1 Psychic 4 Call, there's no where to drop. The Dialga body isn't so good when you have to attack with it without special metals. If it had free retreat it would be the obvious play.
Kyle, we had thought of Banette as well. Mismagius is definitely a better Mewtwo counter though I wonder if Banette could come up better in other random spots. My guess is it still wouldn't be used 95% of the time but I didn't test it. Although we thought maybe the 2nd Mismagius attack could be great in an odd spot too.
I'll respond to the other posts soon, thanks everyone.