If you are racing they are essential. If you are riding brevets they are helpful but not required. If you are struggling to finish brevets within the time limits you are hurting yourself if you're not doing them. Luis pretty much nailed it. If you want to finish brevets more comfortably and with less stress you should be doing some kind of speed work. If you are finishing comfortably mid-pack and your happy with that there really isn't any point is there?
