Never used it personally, but from what I've heard from the coding experts many times, heightfield mapping is really unoptimized (and hasn't been supported at all).
APB once had three DM maps that used heightfield and while it was neat that the mapper could blend many textures together, there were some heavy limitations as well (z-pulling only being one of them, which when trying to make interesting terrain looks bad and doesn't work well for collisions).
Someone else who's actually used it can chime in more.