Ive been using different effects for years. I can usually do 100% of my gigs without effects. In fact, I fought using them for years. But they're a nice option to have. Chorus, flange, phase shift, gates, eq, distortion, wah wah, compression and many others. They're all useful for bass. Once you start to fool around with them in live settings, you'll find slick uses for them. The key is to use them appropriately, sensibly and sparingly. Don't forget, you're still the bass player and you have a function, so don't drown your bass in so much overdrive, wah, delay and other unnecessary self-indulgent effects that your drummer can't lock with you.
I tend to lean toward a little eq box for extra bass since Im playing a 4 string. Compression and gate filters for extra punch. The others are for personality and character. There are alot of amp modelers out there too that give extra dimensions for overall sound. Im toying with the idea of getting either a Line 6 Pod or a Boss multi-effect floor model. Korg and Zoom make nice bass effects too.
While not necessary, the effects and modelers are a nice addition to a bass players repertoire. Every bassist should consider having a nice multi-effects unit at their disposal even if you never or rarely plan on using it. At least, an eq and/or compression unit if nothing else.