That's another mith: Legendaries are cheap.
Take, for example, Dragonite. It's stats are on par, if not slighty higher, that those of most legendaries, and it has a wider movepool.
Tyranitar, Salamence, Metagross and a new D/P Pokemon suffer from the same thing.
Did you ever though they where cheap? No.
Then, take Registeel: It has good defenses, but lacks any attacking movepool, and those defenses are not on par with it's HP. Plus, it's weak to Earthquake and Focus Punch, 2 very common attacks, and it can be surprised with a Fire attack.
Now, take Gyarados: Great attack and Special defense, average Speed and Defense, and not very good Special Attack. It's trait is intimidate, that helps with it's defense problem and can make your opponent switch out, giving you a free turn. It can use Dragon Dance, allowing it to outspeed almost everything and kill almost everything. It has the disadvantage of being very weak to Electric, but it sometimes survives an attack, and then uses Earthquake.
Which one is more powerful and useful? That's it, the non-legendary.
Keep in mind that most legendaries, specially the ones member of trios, have pretty common stats, and no one of them have any stat with over 125 as Base (compare with Alakazam's base 135 SpAtk). They suffer from movepool problems, making them have to use Hidden Power if it's of a good type.
Then, there are the Ubers: Ubers are Pokemon that take variety out of the metagame, due to it having to be about themselves and some other limited things. The Ubers ARE Mewtwo (very good stats for sweeping, and infinite movepool), Mew (Movepool), Lugia (Invincible with Recover and Calm Mind), Ho-Oh (massive special defense and huge attack), Kyogre (special sweeper and tank), Groudon (Physical sweeper and tank, and a GREAT team player), Rayquaza (Salamence Deluxe, able to sweep in both sides), Deoxys (fast Rayquaza), Deoxys-Atk (Outclases all other sweepers, incluiding other Ubers), Deoxys-Def (Doesn't die) and Deoxys-Spd (Balanced and always goes first). Wobbufet is also Uber, due to it being lethal and broken in good hands.
And you can get Wobby in any game since Gold/Silver... ¬_¬