So I was reading a book that I found in the Ragged Flagon Cistern called "shadowmarks" which detailed what every shadowmark meant. I came across this one page that said "This shadowmark represents someone who is under the Guild's protection. You may not steal from them" or something along those lines.
Anyways, this got me thinking.... since they're under our protection, then shouldn't we receive miscellaneous quests to actually PROTECT them once in a while? By that, I meant that we should be given radiant quest objectives like "eliminate the bandits attacking this place under our protection", sorta like how the companions have that radiant quest objective to exterminate pests in a house for one of their clients.
In my opinion, I've always seen this as a possible oversight when Bethesda made this game.