How would shutting down the machine be bad? Real question
It is reckoned that the switching on and off cycles stress components.
Think of an incandescent light bulb, for instance. Left turned on they last for a long time (many years). The heating and cooling cycle of switching them on and off reduces their lifespan. However, switching off is more economical because the cost of extra electricity used would far outweigh the cost of occasionally replacing a light bulb.
However, I doubt that the analogy would carry through to computers, which tend to be left on for long periods. Light bulbs are often switched on and off several times a day.
With the power switch being a little awkward to get at on Mac's, putting them to sleep seems favourite. A sleeping Mini uses little more than 1 watt of electricity.
Even when switched off, a plugged in Mac mini still uses about 0.2 watt. At idle it kicks in at about 10 watts, and the monitor adds to that.
Here is what Apple has to say in the 2009 Mini manual:
Putting Your Mac mini to Sleep or Shutting It Down
When you finish working with your Mac mini, you can put it to sleep or shut it down.
Putting Your Mac mini to Sleep
If you’ll be away from your Mac mini for less than a few days, put it to sleep. When your Mac mini is in sleep, the display’s screen is dark. You can quickly wake your Mac mini and bypass the startup process.
To wake your Mac mini, press any key on the keyboard or any button on the optional Apple Remote. When your Mac mini wakes from sleep, your applications, documents, and computer settings are exactly as you left them.
Shutting Down Your Mac mini
If you won’t be using your Mac mini for more than a few days, shut it down. Choose Apple () > Shut Down. To turn your Mac mini on again, press the power (®) button.
My own 2009 Mini has seldom been switched off in the five years I have owned it. Typically it is weeks or months between shut downs. Occasionally I take it to work; then it is shut down several times in a day. I have the display and computer set to sleep after 15 minutes of non use (System Preferences > Hardware > Energy Saver). It still has the original HDD, and has an additional 4GB of RAM (5 GB in all). Start up takes a minute or two, whereas wake up takes just a moment.