Not a big deal if it is mains powered, but it is if it is battery powered.

Which brings up the point that the battery won't last long if it's powering standard 555's continually.

 

And why in the world make the thing so complex as adding an FET to the circuit???

It's to isolate the LDR from the 555. It reduced the standby current and the alarm shuts OFF immediately when the door is closed.
I would like to see your schematic using just a cap, resistor and LDR with a 555 to do the same.

OR, consult one of those "555 Timer" Cookbooks that contain exactly the circuit to do the project

Not required. The circuit the TS posted is the exact circuit using 555s to do the job.

 

Another version using the LDR to turn on the 555.
Standby current appx 25ua when dark.
These are the numbers I recorded using a generic LDR.
Delay appx 23 seconds.

 

Why do you say that?
The timer output is digital low impedance and can drive a 200mA load, so its output voltage will only vary slightly with that load.

What I meant was that in the circuit of the thread starter, the timing in-series resistor and capacitor gets its voltage from the output of the timer IC (pin 3). So the voltage applied to the series resistor-capacitor is removed when the voltage across the capacitor has become great enough. But I tried this configuration at my bench, and as far as I can tell there is no problem doing it this way.

Below is a circuit that I designed and tested minus the buzzer as I don't have the type of buzzer needed for this circuit on hand.