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.
4) You don't ONLY have to go on dates on the weekends. We're all busy but you can definitely manage some quick dates during the week so that you don't only see each other once a week.
5) You'll see what their intentions are. If you don't "take things slow", you will get a much better idea of what their intentions are rather than if you see them only once a week.
6) If things are going well it's a GOOD thing. It seems that people mainly start saying they want to "take things slow" when their new relationship is actually going well. If there's a connection, why risk losing it.
7) It's not going to help you avoid meeting someone "who just wants to sleep with you". If you say you want to "take things slow" so that you "make sure" the person isn't in it just to sleep with you.......well you're in for a reality check because when someone just wants to sleep with you, time doesn't affect that in any way.