Watching L2 is like playing poker. To say it is not helpful is not accurate for everyone. It may be helpful to you, it depends on how good you are at figuring out who is showing the opposite of their intentions (bluffing!). Not sure if you can teach this, but the more you watch L2 in your stock, you will see some patterns, for instance you'll recognise INCA pegged orders (ie wait for the stock to run) or MMs stepping in (ie don't trade against MLCO, they always come in with alot in reserve), for example..
I think more likely to believe MMs, they don't fool around as much as the ECN players, but ALL are using reserve, which is the main tool used to hide your true intentions.