1) You're wasting your time. Why would you not want to see the person you're dating more than once a week? If anything you're just going to waste more time because it will take much longer to see if they're a good fit for you or not.
2) They may take it as you not being interested. Saying you want to take things slow could be seen as a cop-out, so this could ruin your chances if you're actually interested.
3) You're giving them a reason to keep talking to other people. If they see this as you not being very interested, then why wouldn't they keep talking to other people in order to find someone who is and SHOWS that they're interested in them.