I used to believe that tiers don't exist. I still do, to some extent...well, it's more that I've changed my view on it than anything. To me, I think what factors into it is an individual character's own natural potential level, and a players skill with that character.
So, for example, I think most people would agree that Fox has more potential to be a powerful character than, say, Pichu. Pichu, when used by someone with high skill, can definitely beat a Fox. Hell, sometimes when all my friends and I are playing top-tier character matches, I'll go YL or Pichu or someone and just rapesies them. However, if Ken were to clone himself and play against another Pichu when he was Fox, I'd bet that the Fox would win. It might be a close match, but the Fox would win more times than the Pichu would.
I think that a person's skill matters oh-so-much more than the potential of the individual character. My best friend foxroar can usually kick my *** with just about any character he chooses. It doesn't matter if he goes Donkey Kong and I go Marth, or if he goes Falco and I go Marth, he'll still be better than me because he is, on average, a better player than me. He can l-cancel faster, he can combo better, he can out-mindgame me, he's just better on almost all levels.
So, to determine how good an individual player is, I'd say that it's a ratio between the potential of the character, how skillful the player is on average, and how skillful the player is with that specific character. Just because Ken's good with Marth doesn't mean he'll be good with Yoshi. I'm sure Gabe's Yoshi would crush Ken's Yoshi; the character has the same amount of potential, Ken is a better player on average, but due to how the ratio is spread out, and since Gabe is more skillful with Yoshi than Ken is, Gabe wins.
That's why I personally feel like me, or most people, maining Marth has been futile. I'd argue that Marth has a huge, if not the largest, potential out of any character to be extremely powerful. However, it is *very* hard to actually be able to utilize that potential; to draw from that well of power. Ken has been able to tap into that limitless supply of mana, and thus he rapesies.
But okay, back on topic...I think that playing a Low Tier character, and playing that Low Tier character well, will make you stand out from the crowd - look at most of the really good Low Tier players (like Chu, Arash, or Taj when he goes Mewtwo). They tend to stand out in a different way than the top players. Playing a Low Tier character, due to the limits of that individual character, might mean that you can never beat certain top-players; if, out of those three factors I listed above, you're even in the second two, and the only difference is how good the character naturally is, then, on average, you'll lose more often than win.
So yeah. All in all, I say just go with whoever you like.