Oh, I love diff3. Having the common base (usually) makes it easier to see "what _change_ happened in the other branch", and "what _change_ happened in my branch". Then it's (usually) straight forward to apply one change to the other.
Of course, if seeing my code and their code works for you, then by all means keep doing what works.
You don't know the entire common base; only small windows of it where merge conflicts occurred. Elsewhere, a machine automatically merged it. That machine prudently had the common base, but didn't understand any of it.
Of course, if seeing my code and their code works for you, then by all means keep doing what works.