The easiest example I could think of at the moment would be block type being flowed on a surface with convolutions in all directions.
Where features within the type could not be simply extruded one way or the other.

This 'type' element could be substituted with other type of geometry.

It's kind of an esoteric action to think of, but in years of using Corel's 2D Artistic Brush with is a "flow along path", various enveloping distortion tools and the fit text to path features - I've run into little problems where type can bunch up into areas, paths invert, and other off phenomena due to the distance from the path exceeding the given space due to tighter radius of the underlying base path.
I havn't had the chance to play with the Flow tool yet, except what I could remember from using the Rhino demo - but I remember bunching and folding of surfaces that were a problem. Not that this IS a real problem. There are a myriad of thing one could do before hand to keep there objects from having unexpected results. And, no, I was not implying that a rigid vertical or directional driven lock to the displacement was a fix to that type of thing.

It would be just an option for a different variation of a theme, or a way to achieve a particular result.

Of course, your new Flow command, from what I've seen the others use is utterly fantastic and full of all kinds of promise... I can't wait to sink my teeth into it!