Since most lights have the whole body as the negative connection, the area not too far of the positive connection is negative. If a magnet is placed on top of the plus side of the battery and you, say, drop you light--the magnet can slip a little and still be touching the positive side of the battery but also the negative body. Then you're shorting your battery, which can get very dangerous with an unprotected lithium ion battery.
Putting a magnet down at the bottom gets around the safety concern. I did this with a washer on one of my unreliable lights, but whenever I would drop it, the washer would short the negative of the battery to the body of the light---thus bypassing the tailcap. So the light would be stuck on until I opened it up to line everything up.
I've found that cutting a screw short and sticking it in the middle of the tailcap spring helped get a good connection without ever bypassing the switch.