Have you ever heard of the saying "patience is a virtue" ... unfortunately, I think side quests are a necessary evil to allow us to gain the experience we need to become big, strong spell casting wizards. That being said, they are completely optional, so you dont really need to do them (except for the XP, gold, and various drops)
Without the sidequests, you would level very, very slowly. As it is, the main quests and side quests given in all areas but Grizzleheim add up to the amount of experience you need to hit level 50 by the time you are at Malistaire. If you did not have access to these quests, your only source of experience would be from repeated runs through dungeons and duels.
However, as has been said, the quests are optional, and you can pick and choose which ones you wish to do.
nah but i agree, there are WAY too many side quests, especially in celestia. but what bothers me more is the "search quests" where you must search for a specific enemy to get something you want. heres an example of what i mean: get 3 spectral energy from shadow-web mercenaires
these are the quests i hate, cause every time you beat them it will say " did not find spectral energy, please try again" and that gotta go one again and again. idk but for me these " search " quests are taking the fun out doing side quests