Nick, M60's come with both spikes and rubber feet.
Your hypothesis about the spikes is basically correct, I think. It's like what Peter said. Using the spikes "couples" the speaker to the mass of the floor, instead of it being "uncoupled" by the isolation membrane that is the carpet. I don't know that I could hear a difference (don't care don't care don't care). However, it also raises the speaker an inch or so, which could be audible and desirable. Most of all, it keeps them in one place - you cannot slide the speakers at all after putting them into the carpet with spikes.
Plus, spikes are just cool. Metal pointy things. Oooh.
