[Lifebot]

That's what confuses me. AJAX loaders are ubiquitous on the web. They're small, easy to use, and a great visual cue. I'm not sure why you would want a percent-based loading bar unless you actually knew how long something would take to load. (e.g. see loading bars for downloads)

Using these super thin bars as a "dumb" loader seems like it would only confuse your users. It's like using an icon other than the floppy disk to convey saving.

[bmelton]

There are step functions that allow you to approximate it or increment it gradually. I might not know how long an individual step in a process will take, but if I know how many steps there are, I can know approximately how much to increment on the completion of each.

This doesn't suit all usage patterns (like file downloading where you don't know the file size), but I can think of at least a dozen scenarios in which this is perfectly suitable (even if I don't personally love the effect).