This question is as simple as saying you want to waste your time on someone that does not want you. To me, I think anybody who understands time won't give such a try. Going after someone who does not want to commit the rest of her with you is simply a waste of time.
Even if you end up getting married to such a person, you will always have issues to trash out. As a lady, insisting on marrying someone who does not want to marry, you will only reduce your worth. The same thing is applicable to guys.
I think it will be best if you can find someone that truly loves you, someone who is ready to spend the rest of her life with you. However, you can embark on the journey with such a person if marriage is not the main objective of the relationship. But this isn't always the best advice for women.
The right answer to this question actually depends on what people want for themselves. But to me, I won't advise anyone to initiate a relationship, when you know your partner won't marry you. It is basically a waste of time and resources. When you consider the totality of what you are going to give that relationship, then you should give it a second thought. Also, I won't advise anybody that needs a serious relationship to do such; it is time-wasting, and you're definitely going to regret it in the long run.
However, I'm fully aware of the fact that the word 'dating' has so much lost its meaning, especially among young ones. For young ones, attempting such is never a problem. Dating people of the opposite sex will broaden your knowledge about what it looks like to be relating to the opposite sex, and it will help you to know more about them.