To be honest, I've never liked the screensavers on my Kindle (probably makes me uncultured, but I read programming books almost exclusively.) Always wondered why I couldn't change it easily - guess this is the reason. $25 isn't a great deal of a saving, 30-40 seems like a better saving, but I'd still choose it, I'd actually prefer to see advertising which may interest me on the main screen, especially if it uses Amazons recommendation engine to show me books that I would like.
I guess the main advantage of this is that Amazon can say things like "starting from $114!" even though many people will choose the ad-free upgrade. I'd also imagine that down the track they'll move it down to $99 ad-supported, which would be great for marketing.