Coincidentally, just yesterday, I bound that command to "C-x v H" in my config, after I realized that I had been doing M-x vc-region-history for a few dozen times.
I always wondered why this command was not bound to the "C-x v" map, and have also created a little patch for master branch to add that binding.
That's very far from my experience. I frequently need to investigate
fragments of code which have very long histories, and "log -L" always
gives me the full picture, even when the code in question changed
significantly, right to the commit that first introduced the code.
Perhaps the secret is in choosing the appropriate region. I normally
try to submit the entire function where the code lives, and it works
very well.