Well this is somewhat true, how do you know they will be active in the clan? How do you know they will post based on their history? Also, if you reject them, they will learn what they should do before they apply again to another clan. Remember rejection is never the end.
However, in my opinion... I would say for you to just give them a trial, see how they do. If they are good, re-evaluate their app again. If they do bad, then you explain to them what they did wrong and show them what they should have done better.
You absolutely can. Granted it's not a very good way of deciding, but it's up to you who you let in your clan. The rule is, 'do not mention post counts'
It's only a problem when people spam because they think a high post count is a free ticket into any clan. Then from the clan's point of view, if a clan accepts anyone just because they have a high post count without actually knowing that person, it's sure to end in chaos. After that you end up with clans that think they can resuscitate themselves by spamming. Never works.