For what it's worth, I think that it really depends on what type of tea you're talking about.

With Black tea, I only steep once because subsequent steepings lose so much of the flavor.

Just the opposite is true with high mountain oolongs, though, which are rolled into tight balls during processing. During the first steeping, the leaves don't unfurl completely, and you have to steep the tea a lot longer to get a full brew... and it ends up tasting a little more earthy, and sometimes a bit more bitter.

On the second steeping, though, the leaves are unfurled and there's more surface area in contact with the water, and the resulting brew is noticeably more fragrant and sweet.

I also find that the first steeping of most green teas is a little bit more vegetal in flavor, or grassy, depending on the tea, and the second steeping is a little better... but it just seems to me to get weeker after that.

Just my two cents

- Matthew