I've heard much the same said of the Dark Brotherhood, but at least most of them have the disadvantage of - SPOILERZ!!!! - gettings the ax through the storyline. Theives Guild, same problem, but with less excuses.
NPCs with unique voices seem to be hard to turn into followers or marriage candidates, with their lack of appropiate lines, so it involves more work than just adding them to a faction, it seems. It even happens to NPCs with generic voices, if said voice types lack the lines. A shame, really, the only faction with a feasible number of followers are the Companions (with both "recruit" and "leader" type members being possible followers).
By the way, to become Guildmaster, you have to complete 4 special jobs (one in each major city apart of Riften). Getting them can get tedious unless you accept then reject jobs from Vex and Delvin like mad until you get the one you want (you have to complete 5 of their jobs per city to get the special one). The Guildmaster armor is worth it tho.