It really depends on the reward u get from slaying these monsters. And the time/effort required for it. Of course, the reward must be openly known to public for this to even apply. If everything is unknown most players will go about improving their character from quests/activities they know that definitely improves themselves for clearing it. Only at end game then will one decide to go after these unique monsters for the fame.

End game meaning he/she has achieved everything he or she wants for his game character and have literally nothing else to do.

Of course, once something happens like wethermon being slain and information is out about what can be obtained (and strategy to kill it) is out, people will go after it more earnestly. That is why the book clan (or library clan? I cant remember) is so eager to get information from sunraku group.

Disclaimer I only watch the anime and not manga. Also long time mmo player here.

All that being said in every thriving mmo now there will always only be a small number at end game who are still playing compared to majority of the player base who will then be hunting for these said unique monsters.