I'm going to respectfully disagree with bobw. So long as the master valve is installed outside (right where all the rest of the valves are installed), then the only issue a master valve creates for servicing is just making sure the contractor is aware of its presence (so they don't start trying to "fix" a valve that's "not working").
Otherwise, I definitly see the benefit of a master valve to reduce the troubles caused by a valve that gets stuck open. I know that half my irrigation system is for drip irrigation, so if something gets stuck open, I might not know it for days.
Here's a quote from the www.irrigationtutorials.com (the internet bible for irrigation as far as I'm concerned) on the subject...
<i>Actual master valves are used on very sensitive irrigation systems where a valve failure might cause enormous damage, such as a steep slope. If the valves were to stick "on" the slope might wash out. No need for them on the average irrigation system.</i>
Otherwise, the only other comment I think I've heard on this issue is something like some people have timming problems with automatic master valve. I don't remember the specifics, but its something about having difficulties getting valves to properly work when you have two of them in series.