Dialga may have weakness to EQ and Close Combats, but that isn't exactly hard to patch up with the likes of Landorus-T, Gyarados, Togekiss.
Don't forget that Dialga is a Dragon / Steel type. That's some godly typing, offensively and defensively. It's the only dragon without dragon weaknesses (Ice, Dragon, Fairy), and it only has two easily controllable weakness in EQ and Fighting-types. Steel-typing grants it many useful resistances as well as a STAB Flash Cannon to destroy Fairies and Tyranitars. Combine that typing with its bulk, and it is one hard tank to take down. It's pretty much Heatran on steroids (without the STAB spread moves).
Dialga has an amazing movepool fitting for an Uber. Its special movepool has pretty much everything it needs to destroy everything (outside of its STAB moves, Aura Sphere, Fire Blast, Thunder(bolt), Blizzard/Ice Beam, Earth Power). It has a great support movepool, too, most notably Thunder Wave and Trick Room. Dialga is one versatile Pokemon, and it's unpredictable what set or coverage moves you will be facing. You can be facing a slow OTR Dialga or a fast offensive Dialga, all-out attacking AV Dialga, a defensive Dialga that spreads paralysis, Specs Dialga, Dialga with Magnet Rise or Substitute, Dialga with Safety Goggles or Rocky Helmet.
Palkia may not have that amazing typing of Dialga, but Dragon / Water typing comes pretty darn close. Its only weakness is to Dragon- and Fairy-type moves. Base 100 Speed with Palkia's colossal offensive potential and solid bulk is quite darn good. It's like Charizard Y, in a sense that it is a hard hitting, bulky special attacker with 100 base speed. Except Palkia has even less exploitable weaknesses (neutral to Rock Slide and Thunderbolt), A LOT bulkier, and does not use up a mega slot. Also Palkia is a lot less weather-dependent.
Palkia, just like Dialga, has an amazing special movepool. They both have similar movepools, except Palkia has a high crit Spacial Rend and STAB Water-type moves that are bolstered further in Rain. It also has support options like Thunder Wave and Trick Room.
You guys are essentially dropping a Pokemon with equal/greater power of Mega Pokemon, without using up a Mega slot.
Now compare Kyurem-B to Palkia/Dialga. Unlike the latter, Kyurem-B has a crappy typing, being weak to Fighting, Fairy, Dragon, Steel, and ROCK SLIDE. Its movepool is decent, but not ridiculous like Dialga/Palkia, with absolutely no support options like T-Wave/TR. It wishes it had 170 SpA instead of 170 Atk, to truly take advantage of its typing and special-leaning movepool, but instead it has to make do with 120 SpA. It fits into our Doubles metagame relatively seamlessly without causing much disturbance.
Kyurem-B is an exceptional case, and I consider it the benchmark for dropping Ubers. Any Pokemon that is better than Kyurem-B should not be dropped, since it would significantly alter the power balance of Doubles.