One of the other reasons that I highly recommend asking is for the same reason as my last post. If you have to plan all the time, you lose time, you build expectations, and it's rare that your expectations will be met. That being said, I know that I am the kind of person that always enjoys flying by the seat of my pants. But I think most travelers can agree that they are here to experience new things, and I can assure you that not every good hostel is in the lonelyplanet guidebook, or even online. Heck, maybe you'll have to stay with a local family for a night because there aren't any hostels with room, but 9 times out of 10 you'll be happier that you experienced something for the first time. When you plan, your mind creates a scenario in your head, and you lose the authenticity. One of the most popular quotes that I've seen while traveling actually goes like this:
"When was the last time that you did something for the first time?"
The last reason is because of the mental tax it takes to always be planning, not to mention the time required, which can all be lost if something falls through. There are plenty of other things to spend that brain power and time on and who wants to be tied down to a strict schedule while traveling.