Well for starters... *clears throat* "Didn't Sakurai in that one interview say 'I guarantee Megaman will be in Brawl and also as a side note, I love wavedashing' a year ago? *provides no reference*" Saying something is great, but linking to a source is better.
Secondly, Sun-Wukong, don't misuse the definition of the word glitch. "Glitch" indicates that a fault in the game engine is responsible for something. That is not the case with wavedashing. The mechanic that allows for wavedashiing had to be specifically implemented by the development team. It reacts exactly how it is intended to. The developers considered what to do when a player air dodged into the ground. They had two options: do nothing, so that airdodging into the ground would be just like doing it into a wall, you'd just kinda slide a small amount in your air-dodge animation, and then awkwardly fall out of it and immediately land, or they could come up with a specific way for the game to handle this. They opted to do the second, and decided to put you straight into a standing position, without losing momentum.
There was no "accident" that created wavedashing. That mechanic was as intended as jumping. It was intentionally and specifically programmed. The use of this mechanic, however, was unforeseen. They didn't, at the time, imagine it would be used the way we used it.
This was, at worst, a programming oversight, but not a glitch. Glitch implies something entirely different. How Jigglypuff stays large if she uses her FS on Eldin when the Bridge is rebuilding? That's a glitch. The game doesn't know how to correctly handle the circumstances. The game knows exactly how to handle air dodging into the ground. It's no fault of the game engine.