whether you wish to. It's personal preference, and for me, I do have an external for storing not just my mp3s, but photos taken, documents created and downloaded drivers/softwares, if I'm heading to a place where I won't need or can't use my laptop, I'll just bring the external hdd. As a backup also is not a bad idea.
I only have 5gb of music on my macbook, which I backed up using a flash drive and that's it. My macbook is used primarily for surfing the web and not much else. I was able to erase my hardrive and reinstall using the osx disc provided to me by apple when I bought my macbook. I came out of it just fine, so I'm wondering if I need to buy an external hd considering I don't have much of anything to backup?