Hardly useless. It doesn't only apply to those who have no concept of God, but also those who do.
An active atheist is someone who actively holds the opinion that no god(s) exist. A passive atheist is someone who holds no opinion as to whether or not any god(s) exist.
A person does not have to be ignorant of a concept to hold no opinion as to whether or not the concept is valid; someone who understands the concept of God can still be a passive atheist.
As such, the distinction between active and passive atheism does serve a function... to distinguish atheists who hold no opinion about god's (non-)existence from those who do.
An apolitical person is still aware of what politics are. Callling a person who did not have a knowledge of the definition of politics would be wrong and pointless.
Pointless, but not wrong. An ordinary building brick is apolitical, atheist and asexual. It might be silly and absurd to describe it by these labels, but not
wrong.
Would you call a person who had no concept of deities an 'unbeliever'?
I'd call them as a non-believer. The word unbeliever adds subtext. Like undead instead of not-dead.
That makes no sense. If you said to someone, "You are an unbeliever", they'd probably say, "An unbeliever in what?" Then you'd have to explain God.
Why would you have to explain the concept of God? They can't believe in something for which they have no concept for, therefore they must be non-believers.
Being an agnostic requires that you understand the position on knowledge, just as being an atheist requires that you understand the position on belief.
The word atheist ultimately derives from the ancient Greek word ἄθεος (atheos), literally meaning "without god". The prefix "a-" simply means "without".
Does a newborn baby have a political viewpoint? If not, it's apolitical.
Does a newborn baby have a god? If not, it's atheos, or atheist.
To describe a non-sentient or barely sentient being or object as having either of these positions is nonsensical.
But describing a non-sentient or barely sentient being or object as lacking a position is perfectly valid, which is what essentially the word
atheist means in this context.
Can you imagine someone using a sentence like this?
"Well, he was a Christian up until the minute he died, but now he's a passive atheist."
No. Aside from the redundancy of "passive", a person no longer exists after death
*. Ascribing a lack of belief to an extant object or entity is one thing, but ascribing a lack of belief to a non-existing object or entity is a completely meaningless assertion.
*Unless you wish to argue that his soul or spirit lives on, in which case you have no reason to assert that they are now atheist without first communicating with this soul or spirit to determine their current state of belief.