The latches are in the middle of the window. So whether you slide the bottom up or top down, you can't reach them. They are designed in such a manner that a tool wouldn't help. (They have to be slid in at a certain angle, then up. It's hard enough standing smack in front of them.)
Yes, we don't have central ac. I leave them open just for ventilation and have never had water come in in the 5 years we've been there. Though I'm selective with which ones (considerable overhang). I am no longer leaving them open when we're not home. (I didn't really think about it until you mentioned it about an unlocked is always easier to break into.) But I like to at night for the fresh air.