"TPMs are still pretty rare in the average user's computer "
I believe this depends on how old the computer is. I'm not certain, but my understanding is that most new machines include a TPM chip on the motherboard. Interestingly, the documentation and salesfolks with at least one manufacturer (Dell) asserts that they don't -- but they do. If you care one way or another, it's probably best to ignore what the companies say and test for its presence yourself.