So it is worth doing all the guild quests? I'm leader, but I have loads of offers still open. I kinda thought they were fillers, but am I to understand that it's part of the quest?
Not essential, but getting those jobs from Vex and Devlin contribute towards restoring the guild.
They each offer small jobs in random cities. Once you have completed 4 or 5 of those jobs in a particular city, you are assigned a Special Job within that city. The chosen job locations are completely random.
For example, you could get a burglary job in Whiterun. Complete it and your next job could be in Marketh. Not really a problem, as you're contributing to both cities and stacking up your jobs completed. However, if you have been given the Special Job in Whiterun and been successful, it is pointless continuing to get smaller jobs in Whiterun as they'll just loop. This mini set of quests is good in that respect, as you can simply tell Vex or Devlin you don't want to finish it and ask for another job, hoping it will be in a city you haven't completed the special job in.
Ignore or pull out of any quests in Riften, too.
The four cities you ultimately get Special Jobs in are :
Whiterun, Marketh, Solitude and Windhelm.
Like I say, once done enough small jobs within a particular city, Devlin will offer you the special job for that city. Once done, ignore any further jobs in that city, they are pointless. Simply pull out of the job and ask for another.
You'll get Guild Master Armour, a fence located in certain areas offering 4k gold to purchase from and other little peeks too. Plus it completes the Thieves Guild completely.