He really isn't. He's a ghost type variant of clefairy. Seriously, compare the two side by side and try to disagree. Here's links to some pictures: Gengar: [link] Clefairy: [link]
Also, there is no reason for haunter to lose his badass clawed hands for a pair of arms that are so stubby that they couldn't even reach an enemy, much less be of any use in combat. On top of that, he goes from a sleek, streamlined shape to something that vaguely resembles resembles the blueberry girl from Willy Wonka. If you remember the movie, all that she could do was waddle, and I don't see Gengar being any different.
Okay, I can agree that he is a lot more powerful in combat, but he still looks stupid, which makes him a poor subject a visual medium. Also, even though he looks a lot more deadly in that picture you sent me, Gengar still looks retarded. Badass retarded, but retarded none the less. If it makes you feel any better, I have been considering the concept of "fixing" the design and maybe posting an altered, better looking version. Maybe.