While the use case of secure boot is often anti consumer/end-user, there are many applications that make sense to have attestation (yubikey type embedded projects, etc)

Without free software implementations of secure boot et al. all this would just happen behind closed doors. At least with this the field progresses and you'll have the tools to secure your own applications when the right project comes.

> What good is having all the source code in the world if I can never put my (or anyone else's) modifications to it into effect?

Well, it'll be more difficult to get pwned, for one.