If you are an intraday trader, then L2 is extremely important for identifying pockets of liquidity, which is crucial for order-execution purposes. If your time frame is longer, then the value of level II decreases, as it is primarily a tool used in an ultra short-term time frame.
Although I'll be the first to admit that L2 can be misleading at times, the other indicators are not much better. The saying that charts and T&S are "reality" is extremely flawed, in my opinion. If they were reality, then there would not be terms like "erroneous prints" and "false breakouts". The fact is, you can base your medthodology strictly on charts and T&S, and still get manipulated and burned on a regular basis by other market participants.
Here's a good example. A stock is close to breaking out to the upside, and I have a huge order to sell, let's say a couple hundred thousand shares. I know that if this stock breaks out, everybody and their brother will be buying it. The stock seems to be hesitating a little bit, and I really need to dump these shares. The last thing I want it to do is to collapse before it breaks out, because it will be difficult for to sell at decent prices with everybody else trying to sell and the same time. So what do I do? I start buying the stock, but in small 500 and 1000 share lots, just enough to break the stock out to new highs. All the "reality" traders using charts and T&S see the breakout pattern form and the positive trades appear, and therefore start jumping in, so I start putting out stock to sell to every technical trader that wants some. By the time I've finished dumping my load of stock piece-by-piece, the stock has tanked well below the breakout point. The Result: All the reality traders lost their shirts and I completed my objective.
Opponents of Level II say there's too much noise for it to really be useful. Although somewhat exaggerated, this is a legitimate complaint. There is a lot of noise in the form of constantly refreshing bids and offers, but that's because it's a real-time display and not a historical indicator like a chart, so let's not act like we're comparing apples to apples here. You can change your chart from a 1 minute period to a five or 15 minute period to filter out the noise but there's no such adjustment for a level II display because its a completely different kind of tool. But that's fine in my book. If your strategy is one that only requires that you get an update on a stock's chart every 15 minutes, you don't really need L2 anyway, except for possibly improving the quality of your executions.
As I said in the beginning, L2 is useful and it has its place, but only for those whose time frames are short enough to make use of it.