Sethbling reliably uses iron door bottoms for force fields in his creations, but (why) aren't these prone to getting updated when Minecraft chooses them at random to make them update, thus breaking them? Is it about the height level of them? Or maybe the gamerule doFireTick?
Here's Seth's original video on the subject:
For some reason, when there's a bottom half of a door, with no door part above it, it doesn't render. Thus, with a checker pattern of bottom halves of iron doors (inserted using MCEdit), you end up with an invisible wall.
The reason that they don't break because random block updates aren't a thing. Random world ticks, such as the growth of wheat, trees, etc are a thing, but the game never randomly updates blocks. If a block update does occur next to the door half (block is placed or removed, redstone update, etc), then it will break. But never randomly (except maybe by Enderman intervention).
Minecraft doesn't randomly update blocks. You are probably thinking of growth ticks, which a block has to register to receive. Doors don't grow, so they're not on the random growth tick list.
A normal block usually receives updates when a block beside it changes and chooses to notify its neighbours that it has changed. The iron door bottom therefore won't usually break unless a block is placed or destroyed beside it, or a fluid changes its flow beside it.