I do all the quests like half of the time (while simultaneously watching some TV, cause otherwise they can bore one into a coma), other time just Wiz and Saggi. I wouldn't mind a higher payout of course, but it's not bad as it now.
I only do quests when Saggy or the Wizard are weekend quests. I don't have the time to do all of them (or don't find it worth my time to do them because it's quite boring).
While on my semi-hiatus for a couple of years I didn't do any quests, but these days I try to do Wizard, Quentin, Sagittarius, and Carl every day to get a roughly 2-2.5mil profit. It definitely doesn't make the big bucks for me like restocking does (which I sometimes spend several hours doing, when I'm in Super Focused Mode), but every little bit helps when I'm aiming for 1 billion sP! Plus, when it's late at night and I'm tired from work, it gives me something mindless but productive to do.
I do agree that it would be nice to see quest payouts increase, though. Half the time, the quests that I've completely given up on don't even reward you enough sP to cover the items you purchased for them. That, plus extraordinary CSC inflation, make it rather hard to save for things like pet slots, battle items or collection items if you're not able to buy CSC with real money. (Which, I get CSC is supposed to be the "premium" currency or w/e, but it feels like it's gotten so much worse the past couple years.) ...Of course, I'm no economist, so what I'm saying doesn't have much meaning, but I figured I'd at least voice my thoughts. :'D

They are super boring but my main source of income haha, well if you include Major Drills. He's always good to me. I usually just sit down with the TV on and do all my quests while watching TV.
My reason to do the quests is also to make profits in a short time. I like to play the games and do other things on Subeta too, but unfortunately I have not much time to do this always. I have got also a real life near the internet. Simple.
Anyway I think the quests are very useful to do especially when I have not much time.