While spikes couple speaker cabinets to the floor, they do so only through point contact. I believe this is much better than sitting them directly on the floor, because the area of contact and thus the transmission path is vastly reduced and the amplitude of the exciting frequency is probably lower too. Nonetheless, they are still coupled, which is not what we want, hence the need to use resilient mounts.
For optimal isolation, we should choose the mounts to suit the mass and exciting frequency. Of course, for speakers we have a full range of exciting frequencies, so we try to damp the worst resonances if we know where they are, or more typically we just buy something that someone recommended.
I have a simple practice for choosing a resilient mount: I pick one that compresses under the weight of the cabinet, but not too much (certainly less than 50%). A small mount under a heavy box will be pressed solid and will no longer be resilient, while a big mount under a light box won't know the box is there. I look for soft polyurethane and experiment until I get the compression about right. I would do this for a concrete floor too - vibration moves in both directions - but each to his own!
