I am very familiar with the type of suppression you indicated. We use them on solenoid vales all the time. The back-back zeners shown make it polarity insensitive, in other words you can wire it without paying attention to polarity. Using zeners will also make the drop out time of the valve faster and more consistent than just with a simple diode. Of course, it costs more. If you are doing induatrial wiring, the suppressor shown is a cheap, simple solution. If you are DIY, a 1n4004 or similar just wired across the coil, with attention to polarity work normally work just as well.
paul