While they don't always give you a 100% indication of where a stock will head after an earnings report, they can be quite helpful in making a trading decision. Last week with $CSCO up against the $26 level I used a long term chart to determine that the odds of a sharp decline post earnings. $CSCO dropped almost 10% and, in my view, looks ready to drop even further.
After the bell on Thursday $ARO Aeropostale Inc, reports earnings. I think the stock will plummet on this report and find support around $10 when the selling is done. I don't think the drop to $10 will come in one trading day, although it might, rather I think the move to $10.50 is not out of the question.
The stock has been putting in lower highs since topping in 2010. Its recent bout of consolidation has finally concluded with the stock breaking lower. I think the next stop is multi-year support at $10 and that tomorrows earnings report will be what helps take it there.