Gravel or no gravel

Lots of folks in S. FL seem to like designing ponds with filtering systems
similar to aquarium undergravel filters. They run pvc tubes with small holes
connected to
there pumps, which then transfer the water to canister type filters. The pvc
piping is covered with a layer of gravel. I have seen many that are crystal
clear,
while others are quite nasty. One aspect that seems good is that the water
is pulled
more evenly from all areas of the pond, as opposed to a drain on one end.

Any thoughts on using these gravel filters vs. smooth bottomed ponds (vinyl
or sealed concrete)?
My pond is concrete with a black diamondbrite finish. I had once added a
layer of river rock
for contrast, but removed it due to the fast algea growth that occured.