Cabbie explained it well. As you say, buying on MACD alone is not a good idea. As most others would do, they are used as a prompt to check the chart in conjunction with other preferred indicators/candles/patterns.
I like to keep those with MACD cross either above or below the signal line on a closer watch list providing the chart looks ok. I've had a few good runners with those crossing below the signal line, but, could just be a stroke of luck. I've never actually sat down and worked out the averages and likelihood of those that go on the watch list that go on to make a decent gain based on the MACD alone. Would be a bit of a time consuming mission and probably not worth the time, as many are false starts/leads based on a lagging indicator.
(20min delay)