Bangkok is the easiest city in Asia to eat well in without any help at all, which raises a fair question: why book a tour? The honest answer is access and pace. A good guide gets you into the stalls with no English menu, orders the thing that is only made on Tuesdays, and walks you between them in an order that makes sense.
A bad guide takes you to three places on the same block that pay commission. We booked fourteen over two trips. Six were worth the money.