The reason stealing an earth would be dangerous is as follows:
1) If you steal an earth from, say, the ring circuit (ie run an earth wire from a socket), then someone needs to disconnect the ring circuit wiring from the consumer unit to carry out testing or alterations, they have, without realising it, disconnected the earth from this light. You should not be able to create a danger on one circuit by disconnecting another. This is against regulations, each circuit must have it's own Circuit Protective Conductor (earth wire), they must not be shared between circuits.
2) In the event that for some reason the earth on the ring circuit becomes faulty, this also means that the earth to this light is now faulty. Then let's say an appliance plugged in to a socket becomes faulty and creates an earth fault. Beause of the dodgy earth, the earth conductor in the ring circuit becomes live. Because of the stolen earth and a fault in the ring circuit, the light's metalwork now becomes live. A fault on one circuit should not make another one dangerous.
3) Even if all earth in the ring circuit are OK, an earth fault in an appliance plugged into a socket would still cause a current to flow to earth, and could still make the light fitting live for a short period of time.
Trust me - it's a fairly common bodge to avoid rewiring the lighting system, but it is not a safe thing to do. If a lighting circuit has no earth - it has to be rewired. This is just one of those sirtuations where there are no 'buts or ifs'
Hope this clarifies the situation.
Paul