I would guess that internal drains are rarely, if ever, used. My thinking is that its better and wiser to get the water off the roof and away from the home in a simple and proven method as possible. Am also thinking of the skill level of the typical low-bid roofer nowadays; an internal drain would require the roofer to also be a plumber. That thought makes me cringe.

Plumbers do plumbing. Roofers do roofing. An internal drain is not terribly expensive but the piping has to lead somewhere: across the lawn to a municipal storm sewer or to a point of discharge on the property. Careful design requires a backup drain (piping, scupper or over a roof edge), in case the main drain pipe becomes obstructed. Water ponding on a roof can get very heavy.