shielding

[nBit7]

The shielding to to reduce the emitted Electro Magnetic (EM) radiation produced by the electronics.  EM radiation can interfere with other EM sensitive electronic devices (particularly high gain radio receivers).

At the end of the day you don't need the shielding unless the Amiga is effecting some other nearby device (such as reducing your TV or radio reception).  I would be surprised if you found the Amiga without the shield doing this (unless there is a problem with the PSU).

[nBit7]

BTW, I had problems with interference with radio even with shielding. the Faraday cage is supposed to protect you 100% only for 'static' fields, I think (unlike EM). so it only limits the EMI

Shielding will only help with radiated emissions.  Conducted
emissions and common mode noise will radiate out of the cables and travel through the mains and into other devices.  This can be helped with ferrite beads (those lumps you see near the connector on most modern cables) and In-line filters.

Edit:
I should also point out that a Faraday cage does not protect against 'static' fields. And EM waves require a changing field (see Maxwells equations).

[nBit7]

actually, thinking about it, my neighbour was complaining that his alarm clock radio thing was emitting lots of humming and had i got any electronics in the room that backed onto his house...

humming is sometimes the windings in the transformer moving due the changing magnetic field inherent in the transformer.