I know I said I was going to discuss Ghirahim in depth, but to be quite honest I really do not feel like doing a large write up at this time. So I will simply condense a few points and go from there.
I've heard from Nintendo for awhile how story driven this title will be and how new characters such as Ghirahim are going to be fleshed out more than other characters in series past. After bringing up what I did earlier in regards to the cutscenes; it is not so hard for me to believe this game will do anything less than what Miyamoto and Aounuma have stated. Ghirahim is being designed to be both memorable, significant, and different. Aounuma stating that his conception was to have a villain that contrasts Ganondorf. So I do not feel he is going to fall into the category of the other one hit blunders. Being either extremely shallow or overshadowed by Ganondorf/Ganon. Ghirahim looks to set himself apart from not only the villains within his own series, but Nintendo's universe in general. How he is received will be up to both the development team and the fans.
Another note, only appearing in one game is not a negative. I know the common train of thought says otherwise, but it is not. The only question is how much of an impact did they make with the role they had? Which is also is affected by how well the title itself performs.
Feel free to nitpick. I will further elaborate on points as requested.
I will give the character the benefit of the doubt, since we really don't know anything about SS's plot. There is a very slim chance that he ends up becoming just as relevant to the Zelda series as the main three. But I think quite likely this is all just hype. Everyone said the same thing about Zant and Midna when TP came out, and they were just one-shots. Granted, arguably interesting one-shots, but nothing worth putting in a legacy game like Smash.
When I think of adding characters, I primarily consider staying power, whether they will be relevant five years from now when the next smash game comes out. For every series beyond Pokemon and Fire Emblem, this system seems to work. And this is where Ghirahim, Zant, and Midna just don't fit.
Sheik and Young Link had already stayed relevant years before Melee was made...five or ten, it doesn't matter, OoT and MM remain the pinnacle of the N64 era and keeping them in is a nod to Nintendo's history. Toon Link was added as essentially an upgraded Young Link, but they wouldn't have done that if he hadn't already made a HUGE contribution to the Zelda franchise, spawning several popular games...so while I hate the **** out of him he could still be in smash ten years from now and still be relevant as a nod to when Zelda was cute and ********.
We know who Midna and Zant are, but they still have yet to appear in further Zelda games like Tingle or Skull Kid or Vaati, and TP, though a solid game that I admire on many levels, just doesn't stick out in the series. Now apply this to Ghirahim. We know nothing about him, he has no reputation except for being an effeminate creep, much in the vein of Zant. In fact, I'd call him gay Zant, because really I don't see him, don't WANT him becoming any more relevant to the franchise than Zant. This is not how you design an arch villain, this is how you design an underling. Unless Nintendo is stupidly optimistic that Skyward Sword will be the next Ocarina of Time, which I highly HIGHLY doubt, there is simply no reason to put Ghirahim in. He has no legacy as of yet.
I believe the strongest influence Skyward Sword will have on SSB4 is the updated character models and the addition of a new stage. No new characters.