Ringo wasn't technically proficient, but he still provided the right feel for the Beatles and you've got to give him credit for that. It's just not something anyone can do after a couple of lessons.
He's often criticised for being a bad drummer, but the Beatles' music didn't need technical drumming, the focus of the Beatles' music is the singing and the guitars, the drums are just there to provide the beat and to sit in the background. All the Beatles needed was a drummer who could provide the right sort of atmosphere and feeling, and Ringo provided it really well - I don't think there's any denying that.
Comparing him to a metal drummer is pointless (and Joey Jordison is far from the pinnacle) because metal does require technically proficient drumming. It's apples and oranges really. He might not have been a great all round drummer, but he was a great drummer for the Beatles.