I agree. If you are seriously planning to marry then you should spend enough time together to get to know each other (though, you can never completely know someone by just dating - lol).
Buuuuut, I would say that if marriage is not near in the future then limit the time you spend together. I think the advice is given because the closer you grow to a person the more more comfortable you will feel around him/her and that could lead to a problem as far as conduct.
Yes there is a section in the Young People as Book about that. A guy and a girl can be just friends but when they are exclusive friends and Best Friends then an emotional attachment is bound to happen and that is what you have to be careful of becoming to emotionally attached to someone of the opposite sex.