John:

DC offset contributes to uneven emission of from the filament and early
failure. In heaters (this started as a heater controller) there is the
added detriment of uneven power distribution. (This from our heater
manufacturer's engineer; we called him to discuss an observed uneven
temperature distribution).

I'm still looking for my copy of the code. I recall the final result
being fairly trick, but I don'e think it's unique enough to deserve
protection.

-- Tom