Although spikes can transmit vibrations to the floor when used with loudspeakers, you have a concrete floor which is ideal and will not propegate those vibrations to the room itself. They can be a problem if the floor is wood and supported by joists which tend to carry those vibrations to other items laying on the floor (like your equipment rack).
You are correct that spikes are intended to be used with carpet to reach the underlying floor, so correctly used in your situation there as well.
Since you check both boxes with regard to what is the proper use of spikes, all that remains is what is the advantage of spikes over soft (absorbing) supports? That answer is your speakers are constantly vibrating, and if placed on a very slippery surface (say, a teflon(r) pad) would move around rather than stay fixed in one location. Any energy lost to movement of the enclosure is energy better spent moving the air in the room. Spikes very firmly locate the enclosure, and I find the improvement in sound quality ... especially bass performance ... is quite noticeable.
I would recommend you continue to use spikes with your loudspeakers. You could consider sorbathane or some other energy absorbing support for your components, especially any that involve mechanical movement (turntables, disk players) or for the entire rack itself.
People whose sound room construction is not concrete flooring might benefit from an absorbing support for loudspeakers, but that isn't the case with your setup.