(long time lurker, infrequent poster)
Basically, I feel it's a wash as to whether it is a benefit or not.
In an ideal situation, the electronics will last much longer if there is no thermal cycling _and_ if the source (power supply) is consistent. So one might be tempted to leave electronic equipment on all the time as a benefit.
But in the real world, the line voltage is susceptible to overvoltage transients and brownouts which both can have a negative impact on the lifetime on any electronics running off the line. So if you power off half the time, you may miss half the transients invoked by thunderstorms, etc.
Another factor in the equation is the mechanical apparatus, if there is any. Most mechanical devices, such as disk drives, will last longer if they aren't running all the time. The constant friction will wear down the mechanical pieces.
So there are pros and cons to both sides of the story. As far as myself, I leave everything on all the time through surge supressors. And I keep backups of everything on my computer, and consider the disks expendible .
