So will the useable smerit get destroy every month if we don't use them.
smerits do not get destroyed (or decay) for regular members unless the member is a merit source member.
Currently, there are only 120 merit source members. See this link:
https://bitcointalk.org/index.php?action=merit;stats=sourcesMerit source members have to spend their source smerits every 30 days in order to regenerate more source smerits. If they do not spend their source smerits within 30days, then the source merits will not regenerate until 30 days after they spend their smerits.