Planting rocks ?

What do you think about "planting rocks" ? For example:

http://www.drsfostersmith.com/produc...&Ntt=planting%
20rocks&Ntk=All&Ntx=mode+matchallpartial&Np=1&N=20 04

Looks like an easy way to incorporate some interest and dimension into the tank,
and also maybe a way to help contain the plant environment and soils a bit. I
bought a few of these and found that they are cast from an acrylic or other
plastic compound, and have a pretty realistic lava rock appearance. The surface is
much smoother than a real lava rock though, so perhaps they would not trap and
hold food and waste as much. Inside is one or two rounded planting areas, with a
few holes in the bottom.

Would one use a conventional soil stackup inside one of these ? Should we expect
the roots to grow out the bottom and into the aquarium gravel, or try to prevent
that ?

George