My speakers sit on carpet over concrete. I'm using the spikes provided by Von Schweikert and I bought some two-piece "discs" that are machined to have small carbon ball bearings between the discs (reducing the contact area). The spikes sit on these. It's very stable. I've found that a relatively heavy speaker will poke through the carpet and even the concrete. The spikes are adjustable so the speakers can be leveled.
One of resellers of my speakers was really pushing some expensive footers that were $5k for the set. No way am I spending that kind of money on footers. Supposedly, they affect the resonance of the speaker cabinet, or some such thing. I don't think my speaker cabinets are prone to resonating, so it sounds like bs to me. Maybe not....
I've had speakers on wood floors and they definitely needed to be decoupled from the floor. I don't see the need when sitting on a slab on grade. My listening room is in a basement.