Clay vs plasitic pots

In the last APD, Karen Randall mentioned the used of putting sword plant 
into a CLAY pot.  I also put plants into pots.  Good idea. But why do 
people usually suggest 'clay'. 

I use plastic pots. I get them for free from the garden store (from the 
recycle pile).  The plastic ones come in many more sizes and colors and are 
light weight.

What do people think are the advantages of the clay pot (over plastic)?

One possibility is that they are porous, so may reduce anaerobicity 
(maybe good, maybe bad).
Another possibility is that the material provides some benefit to the 
plant or to the aquarium  (see Erik's question about clay pots 
and laterite).  Do they absorb stuff which is good for the plant? or 
improves water quality?
Finally, maybe people like the color (to blend in with 'natural' gravels)