Because companies like Yahoo, AOL and MSN are so interested in page views the download process of IM clients is organized to support that philosophy.
Martin has done a splendid job (I’ve come to expect that from him) in showing why Skype’s adoption and use has been so high. They made it simple.
From day one I have always looked at Skype as IM with Voice. While I may look at GizmoProject more like a real phone, Skype to me has been an advancement on ICQ of all things. Sadly, AOL had that and let it wain.