Redirects of the 301 variety are inherently risky, since they are extremely hard to "undo". Browsers and proxies and everything else will (rightfully) cache 301's indefinitely and forevermore, since by definition, they are meant to be permanent. But "forever" is a very long time, and business objectives change, and fashion changes, and political and cultural climate changes... And so we tend to prefer to the 302 variety, even when it's a pretty sure thing.