IMO, and this is just IMO, so just hear me out. I don't prefer wave-dashing. I play to win, and I'm good, don't get me wrong, I am no noob at smash, but I just think it's...*thinks of the word* not cheap, but taking advantage of the game, I guess. This "glitch" or w/e you want to call it (its nomenclature is not the point) extremely benefitted some characters while it didn't really benefit others. Shiek was one that benefitted a LOT. She also was naturally buffed compared to others (sorry Mewtwo!). Anyway, extremely competetive players depended on using wave-dashing. Now that we have Brawl, she got nerfed a bit. In addition, as all of us know, wave-dashing was given the boot. Now, the competitive characters that used her solely because of high tier listing, being very powerful, and wave-dashing advantage, have deserted her for others *coughMetaknightcough*. Wave-dashing has spoiled them. They didn't play the REAL Shiek. Since I didn't play extreme wave-dash frensy (though I can wave-dash), I never went through the withdraw. It took me a while to getting around to liking her again, but she still is good.
My point? I think that the real problem is that the old Shiek mainers (who wave-dashed) don't use her anymore because she might be a little nerfed. Thus they're are lesser people playing her, becoming better with her. This leads to inexperienced players not doing extremely well. I like her tier placement. She's not in a bad spot, but not in a good spot, as well. Think about it: do you want to main a character that's in the bottom tier so that's it's very very difficult to win? Probably not! However, do you want to play a character that's in the very very top tier so that people hate you for playing that character? Probably not! It's awesome when you face an opponent and they get cocky and confident that they're going to win because they have a higher tier-placed character and you end up kicking their ***. It's an awesome feeling, let me tell you.
That's my speal.