Well, I for one really like preview time and as I have posted about in other threads, would not train at a place without it, knowing what I know now. I did my PSF at a place that didn't have preview time the majority of the time (although i usually found time for it) and I learned a ton more when I previewed.
You learn a ton by previewing, +/- dictating diagnoses ahead of time. You are often forced to make decisions and consider things that you wouldn't normally do in the context of a normal signout (because they don't come up or the attending blows by them). Many times I spend a lot of time focusing on something during previewing that turns out to be nothing, and I would never know it if I just saw it at signout. But when I was on my own, I sure as heck would notice it and I am likely to remember more if I thought about it a lot during preview time and then had signout. Education is not being told what is or is not true - it's figuring it out for yourself.
Previewing does take up more time commitment than just going to signout, or quickly perusing cases before signout, but it is very much worth it.
I also think it is important at all stages of your training. At the start of training, it's important because you can spend time on things and learn different trends, associations, etc. And yes, signout can then go fast afterwards but I have never sat with an attending who didn't answer every single question I brought up and take time to discuss anything I wanted to (and I ask lots of questions). As a senior, it is advantageous because you start refining your own diagnostic abilities, and you can see how efficient and how correct you actually are. As for the argument that you can spend too much time during previewing focusing on insignificant things, that's the idea. How do you know it's insignificant? Do you take someone's word for it or do you learn about it yourself and figure out why?
Previewing is a vastly different world and learning opportunity then just observing signout. Sure, you see the same cases and you get a lot of the same teaching, but you don't learn as well, IMHO.
As far as it supplanting time at the scope with the attending, that I disagree with, and that is a potential big gripe that I would raise a stink about. Do they tell you when your previewing dx was wrong? Do they know what you thought? Do they even know that you previewed or do they just trust you to do it? If, as you say, it is a way for them to get signout done in as minimal time possible, they should not be at a teaching hospital, and I would complain about this too (residents don't complain enough).