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You can usually get a fully-functioning certificate within minutes. No need to send in company validation documents.
Because the process is automated and requires no validation from the certificate authority.
Because your company is not validated, these certificates don't help your visitors know who is running your site. If you have an e-commerce site, your potential buyers aren't as likely to buy from you.
The certificates themselves still enable full encryption but there are other security problems. For one, any phisher can get one and can hide their identity completely. Second, they make man-in-the-middle attacks more dangerous. If an an attacker was able to do some DNS poisoning, he could get a Domain Validated SSL Certificate for your domain and redirect visitors to a fake site that allows him to collect visitor information.