I've said it before, and I'll say it again....Trading is part art, part science. I'll leave that open to you -- however you wanna digest that. (i kind of hate explaining and typing alot)
I am guessing and you can correct me if I am wrong, but you don't know how to program and back test? Fast means more false signals, it is like you strapped down to front of very fast motorcycle and you not wearing googles and you along the ocean in Alabama, every bug will hit your face along the path, even those quarter size yellow bugs where you have to scrap for ten minutes to get off the windshield. Whereas if you are going much slower and can see where the swarms are and can filter them out before you get there and go for much less trading than have a much higher percentage of winning trades. Most newbies go for speed and if you want to be still be here, you go for much slower and learn how to trade by using slower. MACD is good indicator as it shows established trends for trend trading and will sure true divergences to stop you from taking trend trades, same goes with RSI. You want longer period moving averages as they produce deeper retracements. Order flow will be useless to you if you can't read charts and volume, also, unless you become very good at chart reading, indicators will take you longer as you don't know how to use them in relationships of price.
Because they work (oh look, someone else already said that). Also I trade with a multi week horizon. How does the state of the order book now tell me what will happen to the market over the next month? If your time horizon is short enough (holding period in the seconds) then the order book will be useful; but probably combined with other indicators. GAT PS Naturally I use the order book in my execution algo