So first, yes you can hold on to your Martial Pool, and if you still have them at the end of the round you can then use them for an attack.
Saving MP is a good idea especially in an even fight or when you are outnumbered. You may want them for Active Defense, which is the most common thing, for Opportunity Attacks for example if someone tries to go into grapple with you, and for other things like say, drawing another weapon, changing ranges, or moving.
If you look at the three combat example transcripts we posted, (which are here on the site and also included in the appendix of the latest update of the core combat rules), you’ll see that saving MP comes up a lot.
It all depends on your own preferred strategy though of course.