If you need a good reason to use a week barrier, here is a big one! I had one bed last year (this year I have two) and I made it, filled it with soil, and planted it with out knowing what I was doing. I forgot (or just plain didn't know better) to put a weed barrier down. It looks like the tree next door really loves my soil!
I had to remove all of my soil from last year and sift it by hand to remove as much of the roots as I could. I then dug as many roots out and put a 20 year weed barrier down. I was going to just use a 5 year barrier but I figured that these roots were pretty strong and hardy so I wouldn't skimp on my barrier.
Here's the pile of roots I pulled out.
I loathe them.