As Netzreport states
Although Gmail is convincing in many regards, it has not found a user-friendly way to handle spam mails yet.
So, what is the problem? One problem is that the spam filter cannot be deactivated and that mails in the spam folder are automatically deleted after 30 days. If one wants to prevent the deletion of legitimate mails, he has to examine the spam folder at least every 30 days.
He finishes with a call of action to persuade google to change their policy.