While it may not be a flashy business, payroll and HR services are something that businesses can't do without. And switching from Automatic Data Processing's software to a rival's would be time-consuming. This helps explain why the company's employer client retention rate was 92.1% last quarter, a whisker away from its all-time high of 92.2%. It grew earnings by 9.8% while revenues climbed 7.5%.Over the last decade, it's delivered $30 billion in cash to shareholders via dividends and repurchased shares, whi ...