This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
Spikes prevent speakers from rocking. They also couple the
speaker directly to the floor. Spikes will pierce carpet.
Some spikes will damage carpet. Most will just put a small
hole in the carpet which is invisible. Putting a heavy
speaker directly on carpet will cause a permanent mark on
the carpet. Spikes can prevent this.
If you have a pretty hardwood floor, then spikes will definitely
damage the finish. A rigid disc under the spike will distribute
the load and lessen the damage. Any coin should work fine. Using
a coin will not change the speaker/floor interaction. Do not use
a coin with a carpeted floor. Alternatives to spikes for wood
floors are Blu-Tack and similar products. (see 12.19)
If your floor is extremely rigid, then the spikes will make
the speaker more rigid. If the floor is more conventional,
such as a suspended floor or a wooden floor over joists,
spikes can have a positive or negative effect, depending on
the resonant characteristics of the floor/speaker system.
The counterforce resulting from a forward cone motion in a
speaker may try to move the speaker backwards, but spikes will
have little or no effect on this. Most audible effects from
spikes are due to coupling the speaker to the floor, so it
will be less likely to resonate on its stand. Some argue that
in most cases, spikes will have no audible effect at all.
Try it for yourself.