Dialga: Not impressed
Palkia isn't useless. It has DPS comparable to Mewtwo and it's bulk is amazing, making it a good option as a generalist, with tradeoffs from Rayquaza. (The Machamp-Hariyama of Dragon types, also press F to pay respects for Hariyama) It's Water double resistance punishes all the common Vaporeon and Milotic in gyms, and allows it to push through Lapras and Walerin with ease.
It also makes its case as a Water attacker, surpassing Gyarados quite easily most of the time, (Unless if weather or double weaknesses come in to play, but then again, who walks around in the rain looking for a raid) as well as being a good rival for the Water type throne.
Dialga on the other hand.... is horribly outclassed. It's double Dragon set is worse than even Dragonite DPS wise in neutral settings, (You can thank Dragon Breath for that) and it's Steel set is outdone by Metagross in almost every case in the limited niche that Steel typing has in the meta. While I understand not everyone may have gotten six Meteor Mash Metagross, Steel typing doesn't have too many uses in relevant bosses, and the Steel set is not interchangeable as a generalist. (Unless if you want to waste TMs)
Even with MC/DM, it STILL lags behind Salamence for neutral damage, and that's not great since you also have to remember that there's Rayquaza and Palkia as well. While its bulk is higher than Palkia on average, (thanks to it's stellar typing) the difference is barely noticeable and Palkia makes up for it with vastly superior DPS. It's moves don't synergize well either, having Dragon Breath, a fast, easy to dodge move and a low motivation sweeper, with Draco Meteor, the Dragon typed equivalent of Solar Beam. (But with 30 less base power) The only niche I could find for it was it being more bulky against certain bosses, like Rayquaza. But then again, would you catch and power up a sub-par Dragon just to last a couple seconds longer?
There's no point in raiding it, only for the novelty of high leveled trainers to see how fast they can beat it with its good bulk and great typing, or if trios or 4 man is possible. I certainly won't go out of my way to raid it, as it'll just be sitting in the box, waiting for the time when I transfer my Garchomps, Dragonites, Salamences, Palkias and Rayquazas. (i.e. never)
Answers
Dialga isn't outclassed by any of the other Dragons. Being a Dragon type that is not weak to Dragon has a good niche in saving your team from getting wiped out from the raid boss and having to relobby, and Dialga still has enough DPS to make it worth it.
Against a Draco Meteor Palkia raid boss, Dialga actually has faster Time To Win than Rayquaza despite having 10% less DPS in general. The reason behind this is that if Palkia were to fire an early Draco Meteor, Rayquaza will not be able to fire off enough Outrages + Dragon Tails to outdps Dialga's Draco Meteor as it will be OHKOed. However, these early Draco Meteors from Palkia help charge Dialga's bar so it can fire off its own Draco, and with low energy gain from Dragonbreath, Dialga will usually find itself not having too much energy overflow. In situations where neither Ray nor Dialga can fire off more than two charged moves, the raw base power from Draco Meteor trumps Outrage, resulting in Dialga having more dps than Rayquaza.
In many cases, Rayquaza is the superior Pokemon. However, when the Dragon-type raid boss threatens a quick KO on Ray, Dialga becomes the superior choice. So far this only applies to Palkia, Dialga (use Machamps instead of Dragons) and Rayquaza (Mamoswine is a better option for this anyways), but it may become relevant in Gen 5 against Zekrom and Reshiram if their charge moves are sufficiently powerful.
