Another factor that needs to be considered when looking at the evolution levels of pseudo-legendary Pokemon is the level curve of the games they're found in.
For example, consider Platinum. Gible can be caught and evolved when you only have 2 badges, and while Gabite is a fine Pokemon when you first get it, it'll be in that stage for most of your adventure. But due to its second evolution being attained at level 48, if you grind a bit in Mt. Coronet you can kick Cyrus's ass with a newly evolved Garchomp and then still have the eighth badge, Victory Road, and Elite Four to enjoy. Compare this to Black and White; the Elite Four's strongest Pokemon are all level 50. That is, the first time that Deino evolves the
first time. Sorry to disappoint you if you wanted a Hydreigon - you'll have to be content with getting pummeled by Ghetsis's (which, incidentally, is 10 levels underleveled, as if to rub it in even more).
But as this is the "little things you like about Pokemon" thread and not the other one, I'd like to talk about my favourite pseudo-legendary instead.
The first point I have for Metagross is that it breaks the mold. Every other Pseudo-legendary is Dragon-type except for Tyranitar, which is still obviously reptilian/draconic in nature; Metagross, on the other hand, is a hyperintelligent, robot-like creature. I feel like in a world where the other pseudo-legendaries were rampaging and causing wanton destruction, Metagross would be built specifically to stop them. Regardless, it's different, and it stands out accordingly.
Second, imagine that you're playing Ruby or Sapphire for the first time. It's been tough, but you've finally managed to fight your way to the end of the Pokemon league, to the final battle against Steven. He opens with Skarmory - not a common Pokemon, but one that you've battled before in Fortree City. He follows up with Cradily and Armaldo - two strange Pokemon indeed, but ones that you eventually realize are the evolved forms of the Lileep or Anorith that you picked up ages ago, tried to use, and then promptly boxed. Then comes Claydol, which hardly warrants a raised eyebrow, followed by Aggron which you might realize is the final evolution of Aron, that Steel-type you encountered all the way back in Granite Cave. All of these Pokemon, even if you've never seen them before, it's easy enough to guess their types and dispatch them accordingly.
Then comes Metagross, a Pokemon that looks nothing like anything you've battled before. You try to hit it, but its defenses are so high that it takes practically no damage, all the while obliterating your team with Meteor Mash, a move you had never heard of before. You at manage to take it down and become the champion, but the question still remains: "Where
on Earth in Hoenn do I find this Pokemon? I want to catch one myself!" But alas, the Pokedex merely displays those two dreaded words "AREA UNKNOWN".
Some time after, you're wandering around Hoenn and for some reason enter Steven's house in Mossdeep City. There, you find a Pokeball containing a... Beldum, whatever that is. But looking at its summary, it definitely looks familiar. And then you see that it's level 5 and can learn exactly one move. It's going to be a long road, but if you persevere, the reward will
definitely be worth it.
Looping back to, and somewhat contradicting the first point I made in this post, I don't think there's anything wrong necessarily with pseudo-legendary Pokemon being either explicitly restricted to postgame or made impractical to use during the story. In fact, I think it's great design for the champion to have that one awe-inspiring Pokemon that you've never seen anything like before, thereby making you want to get it yourself and encouraging postgame exploration for it. Although this happened as well with Garchomp, Dragapult, and Hydreigon on the teams of champions or champion-equivalents and then Dragonite and Salamence used by final E4 members, Metagross has the best execution because it's guaranteed to be the first time you see any member of its evolution line at all.
In conclusion: Metagross's first reveal on Steven's team is terrifying, and when you finally get the opportunity afterwards to try to raise it you're stuck with the near-useless Beldum and the below-average Metang for ages, but when it fully evolves, it is fully worth the effort.
(Also: Everyone who found a Bagon in Hoenn on your own without looking at a guide, raise your hand. Yeah, me neither.)