I don't even fast-forward through opening credits on TV shows that I've seen a million times.
Now this I really don't get. That's why God created VCRs and DVDs. That, and he gets bored of reading his name on the credits, repeated ad infinitum...
... sometimes you sort of enjoy it as a whole, but if you can get through the rough patches a little quicker, it doesn't hurt.
Yeah, I can dig this. It's perfectly possible to like bits of something but not the whole thing. So going fast through the dull bits lets you enjoy the good bits while still retaining an understanding of the overall plot.
There are really good shows and really bad shows. The former you wouldn't watch sped up. The latter you just wouldn't watch. But there's a lot in between that you might like bits of and so fast-forwarding past the dull bits makes sense. For instance, whenever I watch Matrix Reloaded and Revolutions, there are big chunks I skip past and still enjoy watching them overall. Being able to do it on x1.5 or x2 would let you do this while still retaining knowledge of the plot.
Let's face it, a lot of shows and movies have bad pacing or dull bits. This just lets you take charge of re-editing it to fix that.