I don't know much about purely finitely additive measures, but since no one has answered yet let me give my interpretation of the definition given by Yosida and Hewitt in their paper Finitely additive measures (1952).
As you expected, a (non-zero) purely finitely additive measure is clearly not countably additive. If it were, taking $\nu = \mu$ in the definition would imply $\mu=0$.
However, it seems that the right intuition is that purely finitely additive measures are those which are very far from being countably additive. Therefore, not being countably additive is not enough. As Yosida and Hewitt put it in their article, purely finitely additive measures are in fact "as unlike countably additive measures as possible".
Here is my (quite possibly wrong) interpretation of what they mean by this. After giving the definition of purely finitely additive measures, the authors prove the following theorem:
Theorem 1.15 (K. Yosida and E. Hewitt, Finitely additive measures, 1952):
Let $\phi \in \Phi$ have the property that for certain countably additive measures $\psi_1$, $\psi_2$, the inequality $\psi_1 \leq \phi \leq \psi_2$ obtains. Then $\phi$ is countably additive.
Here, we have fixed some set $X$ and an algebra of subsets of $X$, and $\Phi$ denotes the set of finite, signed, finitely additive measures defined on this algebra. For the following discussion, let me focus on positive measures. Then, if $\phi$ is a positive finitely additive measure, we have $\phi \geq 0$ where $0$ is the zero measure which is obviously countably additive. Now, if we had $\phi \leq \psi$ for some countably additive measure $\psi$, the theorem above would imply that $\phi$ is in fact countably additive. Therefore, if $\phi$ is a finitely additive measure that is not countably additive, there are no countably additive measures greater than it. However, we might still be able to approximate $\phi$ from below by countably additive measures. That $\phi$ is purely finitely additive means that you cannot approximate from below either: the only countably additive measure below $\phi$ is $0$!
Another way to say this is that, with respect to the natural partial order on measures, a (positive) purely finitely additive measure is one that is not comparable to any countably additive measure except the trivial one.
To recap, if $\phi$ is a positive purely finitely additive measure, there are no countably additive measures above it and the only one below it is the trivial measure. In this sense, we could say that $\phi$ is "far" from any countably additive measure. Let me say again that Yosida and Hewitt state very clearly that purely finitely additive measure should be thought of as very unlike countably additive ones, but that the explanation above is my own (nonexpert) interpretation and should not be taken at face value.