OK, so you agree that there is a POSSIBILITY that she may change. You and I can debate how probable this change will be, but the fact remains, there is a chance.
Now, I believe IT IS important for him to understand her motivations for engaging in such behaviour. However, you are right, it may not be "worth" his time if he is not serious about her. She may be "too much trouble" for him to "bother" with such a woman.
However, he may not even need to figure her out. It could very well be that she has changed and that's that. The only reason he needs to engage in this sort of reflection and Q and A is FOR HIMSELF - to satisfy that gnawing feeling and discomfort that sits at the bottom of his stomach. He may wonder from time to time if she is faithful, so really, it is for himself that he must do this.
The bottom line is, if he is really into her and wants her, he'll go through this mentally exhausting process for his own peace of mind. However, if she nothing but a mere piece of tail, he'll do her and leave. Simple as that.