Spikes definitely alter the coupling between speaker and floor, the type of change, the effect on sound and whether it's a good thing or not probably depends a lot on your floor construction and speaker placement.
I've got a pair of ProAc D38's that I've had on a carpeted floor in an old weatherboard house on piles. The cones are quite wide and I doubt they would make it through the underlay to the floor beneath which may explain why I can't hear any difference with or without them. We've just moved to a new place with far brighter acoustics and hard floors, so I'll have to experiment again and see what works. For me, speaker placement is the first thing to sort out, then try the spikes. Good luck and enjoy the speakers! I'm just a wee bit jealous ;)
Cheers