I think the more quests the better as long as it has an end goal.
For example a side quest in FFX is pretty much kill all monsters 10 times. If you capture a lot of the same species a mega monster is created then you get x99 of a certain item for synthesis and ability purposes, in addition to a fun fight. (If you aren't max)
Surer killing all the monsters 10 times can be boring, but the reward is very fun.
If they did this for a single monster and not for all, then many people would be disappointed.