I can think of few reasons as why new members are hoarding their sendable merits and don't use them.
The first and most obvious is that they are not a well established members and they are here just for their own interest and thus sending merits or leaving feedbacks, whenever they spot a scammer, is not a priority for them and leave it for higher ranked members.
Second reason, they don't know how the system works, maybe they think it will be retrieved from the total of their received merits.
Another reason, they are just selfish or hoarding smerits for future uses like boosting an alt or a friend's account to rank up.
A reminder:
So I noticed it says unused Merit can be decayed at some point in the future - What's the point of this? Is it just to keep people from stagnating their Merit and not using it?
There is currently no decay,
but if hoarding seems to become a problem, I might add it....