Uneven development is very common when doing standing development.

But some possibilities are undeniable. When you use highly dilute developers (Rodinal is especially good for this, according to my own experience), for a long time without agitating, areas of the negative that received a lot of exposure will have the developer that's adjacent to the film plane exhausted pretty quickly.
And areas of the film that did not receive a lot of exposure will have the developer last for a pretty long time, improving shadow detail (and subsequently film speed).
I have seen prints from Tri-X negatives exposed at EI 25,000 processed this way, and the results were acceptable. Not great. But acceptable.
Developing this way will 'even out' differences in exposure and density from frame to frame on a roll of film. Overexposed frames can live pretty happily next to underexposed ones.

You can also gain some edge effects where areas of high density are directly adjacent to areas of low density. Sharp shifts in tonality, along an edge, will get this 'super sharp' appearance. Some like it, others don't.

It's something that isn't for everybody, but that can yield some pretty impressive results with enough practice. See Steve Sherman, for example.

- Thomas