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I don't remember why, but a few months ago it popped into my head: You know, when I was a kid in Harlingen, there was clover everywhere. We'd pick a bit out of lawns as we roamed the neighborhood and chew on it. It's been decades since I've seen a neighborhood like that...Clover was not considered a weed until the 1950s, when 2,4-D became the herbicide of choice.
Before then, seed companies intentionally added clover into their grass seed mixes. Clover will grow in nitrogen-deficient soil because it can fix nitrogen from the atmosphere. Grass cannot. If you wanted to avoid bare patches on your lawn, you needed the clover.
For example, the book Planting and Care of Lawns (1931) recommends planting lawns north of 35 N latitude with a mixture containing 5% clover: http://books.google.com/books?id=_vL1mNCfVDAC&pg=PA8
For the greater part of region 1 the following mixture
has proved quite satisfactory: Kentucky bluegrass 17 parts,
redtop 2 parts, white clover 1 part by weight.
Once 2,4-D became popular, anything that it killed became regarded as a weed. This included clover, so clover became a weed.