bigfrog,
As a mad scientist who has built two ponds and has two others under care, I would never use anything through the dam for drainage, overflow, etc. only as a last resort.
Natural spillways cost far less, will never leak or clog up like pipes, will never rust or otherwise decay, and require very little to no maintenace. I have ponds as old as 75 years with natural spillways that have never been even maintained...cows keep the growth in check.
There are certainly situations where a drainpipe may be required due to unavailablilty of a suitable location for a natural spillway...but IMHO, folks are far too quick to routinely recommend those through the dam drains when natural spillways are readily available.
As Dave said, you can use siphon systems to drain if that becomes necessary.
