You have to wonder why software even bothers with a progress bar if it's not going to give you any useful information. You're thinking about it wrong. The purpose of the progress bar is NOT to give the user any useful information. It's to keep the user from rebooting or losing their temper, which is helpful to the software provider. Consumers will put up with a lot of things, if they are dressed to look as if they are good for us. This is the latest from Tree Lobsters.
Progress bars are great for showing there has been SOME progress, no matter how small, since you last checked on it.
Those which scrob back and forth remove the entire utility, and make it impossible to tell if a process is just going slow, or has crashed and likely will never complete.
Several times I've left long-running processes going overnight, only to come back to the same screen. A progress bar can allow you to spot such trends earlier, before investing nearly as much time in the process.