I'm not really a fan of the speech tree, all it really does is saves you gold, which is not at all difficult to come by in Skyrim.
Not sure why you didn't pick up Assassin's Blade....attacks from stealth are going to be your bread and butter.
Personally, I went into the speech tree with my current character for two things:
1. Merchant gets you access to items that are stupidly rare otherwise because of whateverness.
2. After I've become Guild Master of the Thieves' Guild, I want to go around and buy a stake in all of the businesses I can. In real life it'd be a way for the TG to sorta go legit and still make money, which is one reason why the mafia (which is pretty much what this Guild is in Skyrim) set up casinos and all that.