I'm not a huge fan of contributor agreements, but at least the "Harmony Individual Contributor License Agreement" looks pretty reasonable and potentially innocuous. It's got reasonable options that limit the company in terms of which license they can choose to redistribute the code under. You promise that you've got rights to the code and that you can't sue them (or any of their sublicensees) for any patents to the code you contribute.
Anyhow, I'd be happy to sign such a thing (modulo which options were chosen and my level of trust in the company). I'm not sure that I'd bother, if I didn't have strong desire to contribute to a given project, but that's another issue. As you say, I don't think it's necessary, but I can certainly see that companies large enough to be tasty targets of lawsuits might prefer to have some documentation that contributors claim to have the rights to the code they contribute, and to be willing to contribute said code under a given license. And keeping the flexibility to relicense can be a *very* good thing.