See Also: Canon Lenses
If Canon has discontinued a lens, there is probably a better lens available. But, available at a higher price. Price is the primary reason to consider buying an older, used lens over a new model.
A new lens model does not change the older model in any way, and the older model likely has captured many prize-winning pictures in its time. So, if you are budget constrained, an older used lens may be what you need right now.
Aside from buying a lens that has a history, the clock is ticking on a discontinued lens' repair lifetime. When the parts are gone, your in-need-of-repair lens will likely become valued only for its parts.
More Reviews: Canon Lens Reviews | Canon EF-M Lens Reviews