RPi boots when usb device plugged in

I came across this interesting post today which shows a Pi (powered but shutdown) will boot when a wifi dongle is plugged in.

Really?
Will it work for me?

I first tried it on my "old" (delivered in May 2012) Model B with 256 MB of RAM and it just didn't work no matter what usb device I plugged in (I tried three wifi dongles, 2 different usb keys, a SDCard reader, a powered usb hub and the usb plug from my X-Mini speaker).
No sign of any green light from any of these.

A bit disheartened at this stage but I still tried my new Model B (512 MB RAM) and this time it worked........with nearly everything.

The Pi booted without a murmur of complaint when I plugged in each of the following:

Wifi dongle MicroNet N150
Wifi dongle Belkin F5D7050 with zd1211 chipset
Wifi dongle Linksys AE1200
Usb from X-Mini speaker
Belkin powered usb hub
PNY 1GB usb key

However, it didn't boot when I plugged in 

Dreamgear SDCard reader
Any of a range of HP and Kingston usb keys.

Interesting certainly, but not sure if I'm ever going to find a use for this finding.
It's just so easy to flick the power switch on and off.