Clay is used for bricks. Bricks are a basic building material. A basic building material should not be worth diamonds. Period.
This, I was on a SMP server for 2 months back in Jan/Feb 11 and people NEVER built brick houses and would trade a ton of cash for bricks. It looks so good as a building material too, it should be common.
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It's still the hardest basic building material to come by. By 'basic building material' I mean items that have no use other than to build with. Which includes stone, dirt, sandstone, etc.
There's no reason it should take you hours or days to find enough clay to make a small building.
And even in 1.7 world gen you could randomly get tons and tons. The world I was playing 1.7.3 I had a chest full of clay. It's just more consistent now. I like random worlds to have predictable patterns.
Yeah, it's also more common in swamps and in ponds in forest biomes. If you go to shrubland, desert, ocean, etc, you will see no clay at all. I don't know about plains, since usually the rivers are kind of deep. So I don't think it's too common; I almost wish there were different colors.
Realistically, clay is found mixed in with sand in lakebeds.
Clay is actually still a little rare considering how much their is in real life. But it's rarity now is fine; its not an annoying abundance but also isn't something you have to spend 2 hours to look for to get a whole stack of bricks.