Is Spiking Necesarry?

mine sound noticeably better when I push the spikes through the carpet to make contact with my basement's concrete floor.  i do this with the subwoofer too.  
pain to move around but the sound is better.

on the other hand i was in the vinnie rossie room at AXPONA and he demoed his system with the harbeth's resting on top of the end tables.  
still sounded very good but a little loose / boomy in the bass if you weren't dead center.  

short answer- yes IMHO

Only if your guests are having trouble loosening up.

Years ago, I brought home 3" thick granite surface plates. I placed my speakers directly on them without spikes.  Floor was concrete slab. Sounded great so I would not say that spikes are always needed.

movement of the entire box back & forth will modulate the cone movement - causing phase error and other horrible things

so you want a way to prevent that - spike or very heavy cabinets will work

or... buy electrostatics or Maggies - problem solved (and a lot of other problems are solved too)

Are the speakers on carpet or flooring? One option is to decouple the speaker from the floor by placing the spikes into isolation bases.
See Herbie's...
http://herbiesaudiolab.net/base.htm

or Decoupling Glider...
http://herbiesaudiolab.net/spkrfeet.htm