But commit messages can easily get plowed under in some annoying "reformatted, I hate tabs/spaces" commit. I do like to see some "why" sprinkled in here and there that will survive those accidents. But I will nonetheless prefer blame output if it is still meaningful. Also, I enjoy a commit that removes redundant comments almost as much as one that removes redundant code.
You can either pass a commit id to git blame to see changes before the reformat happend or you can use git log like this:
git log -L0,10:file.txt
There's alternative to in-code comments answering the question "why" - it's commit messages. They can't be out of date.