I think some people have trouble with the flow of the developer through the cage causing streaking on the negatives. I always make sure to use a developer and a dilution that take long enough for that to be less of a problem (8-10 minutes minimum) and I make sure to agitate with a swirling motion to get the developer going in all directions through the cage. But you definitely need to have the zig-zaggy band in place if you're going to be swirling it, or else the film will all slide out!

It also takes a bit of time to pour the chems in and out, which is another reason not to use a short development time...

Duncan