Allow me to expand on SL's gravity comment.
And on the oxygen comment.
You have already heard why watering from the bottom is discouraged.
Here's why I water from the top.

Gravity;
Done properly, the water will drain through the soil.
As it begins to run out the bottom, it creates a suction that draws air in behind it.

Oxygen;
Done properly, the water will drain through the soil.
As it begins to run out the bottom, it creates a suction that draws air in behind it.

Done properly, watering from the bottom, does no harm.
The problem being, it is more difficult to do properly than top watering.

Jus' 2 cents.

Weeze