You're correct that data rates are not consistently high (although you will get bursts of traffic with significant rate), but a higher bandwidth channel allows you to reduce your latency as you can transmit your ethernet frame faster (even if it contains little data, the rest might be packed with random data).
You misunderstand what I'm saying-- I'm saying that the throughput of a channel that's a few kilohertz wide isn't high, so it must necessarily add some delay to messages (making the advantage from HF over optical fiber smaller).
