The only thing joining the thieves' guild will do for this quest is mark the location of the sewer entrance on your map, which you can find anyway by exploring the Rivertown sewers. However, without joining the thieves' guild, you can not complete the holy items quest, or Corinna's summoning quest (plus, you can get two free levels of Lockpick there; minor spoiler ~[color:"#304154"]one by joining, one by loosing a bet[/color]~ highlight the text to read). I'd keep an eye out for a ring or amulet with a bonus to Pickpocket.