There are several problems with the use of the word, but the two biggest are that it casually lumps together very divergent beliefs, and that it gives the impression (which it is perhaps intended to do) that the really important divide is between "theists" and atheists.
This is something like going back in time to the American Revolution, noticing the many competing political theories, and then deciding to divide them into anarchists and "archists". Never mind that anarchists were a very small faction that never really got off the sidelines. Never mind also that monarchists will not be interested in refuting arguments against the merits of a republic, and republicans will not be interested in refuting arguments against the merits of a monarchy. Neither group will really think of themselves as "archists" or dedicated to some generic "archy", the alternative to anarchy.
The same thing obviously applies in religious disagreements. It may be doubtful whether Christians, Jews, and Muslims should be lumped together as generically monotheistic, but it is clearly nonsense to lump monotheists, polytheists, pantheists, and the many other variations of "theism" together for no better reason than that they are not atheists.
In addition, atheism needs to be defined, and then we can ask whether they are quite as distinct as they want to believe.
If there is a main idea to "theism", it is that there exists at least one being to whom latria is due. Atheists of course deny this. However, it is not necessary that atheists deny the existence of beings to whom dulia is due. An atheist might conceivably believe that Jupiter is a real being, and one that is worthy of veneration -- but not of sacrifice.
In fact, I suspect most modern atheists actually think the universe as a whole is worthy of veneration, only not of sacrifice or outright worship. This makes them not unlike pantheists, who think the universe as a whole is worthy of worship. As a result, most atheists could probably be called irreligious pantheists. I suggest that atheists and pantheists should probably be grouped together in any organizational chart of religious belief.