I always thought the Dark & Steel type were brought in to counter what have been otherwise uber types (Ghost, Dragon and Psychics). The rationale while acceptable wasn't that important.
Steels make better sense 9000 times more than Dark though. Steel cannot be broken through unless you change it's form by force, either chemically (Fire) or through outstanding physical power (Ground, Fight).
What I don't understand is how Dragons are uneffective against it, it merits at least a neutral effect, since Dragons are by nature brute physical forces and can learn all natural elements, specifically fire moves.
But balancing acts are always tricky and this had to happen.
The Dark type however is confusing to say the least. Actually the whole concept of good and evil is confusing in pokemon world.
If Psychics are good, shouldn't THEY be the ones hurting Dark types? Furthermore, if they are neutral good/bad wise, why should Dark hurt them? It should be nuetral. Or for that matter, why should the undead Ghosts give a crap about Darkness when it's their natural habitat? Maybe Psychics are just pure who have outstanding special abilities. Pure doesn't do well against evil types.
You can only make sense of Dark type if you think of it this way:
1) Psychics and Ghosts are OP as fuck. Proceed --->
2) Vaguely imply Psychics are the good type but sissy type who can only manipulate minds (Fight, Poison) and don't do physical fighting, so something evil must destroy it, like ghosts.
3) But ghosts are evil AND OP.... ghosts can only scare Psychics because they can posses them and mess with their minds and know each others weakness and can bypass fighting types due to their physical/lack of nature. DARK IS MORE EVIL, there, solved, two birds one stone---> Employee promotion
4) But eh what about Fight type, aren't they the good guys?
5) mhm they can fend for thereselves.
6) But wait wouldn't that make Dark just a new uber type?
7) *floods pokemon world with fighting moves*
