I played fretted forever, always curious about fretless, but never secure enough in my musicianship to try it out. Finally, last year after watching the Cream reunion DVD, I walked right down to my local music store and ordered a Spector fretless, sight unseen.
I found out that fretless isn't that hard, and that my ear is much better than I thought. I was actually playing out of tune more on the fretted, perhaps because I'm more aware when playing fretless?
A funny thing happened as I played fretless. I found my sound, my voice. The way I have been trying to make my fretted basses sound for over 15 years came out instantly on fretless! I found my technique translated very well to fretless. I am playing fretless almost exclusively now, and have yet to be fired from a band, nor have I not been called for other gigs.
To sum up my point, no you don't need a fretless, but if you have the means, try it out. It could be a nice addition to your playing, it could be your true voice, or it could be stuck in the closet or up for sale on TB in a matter of weeks!