I prefer seeing the depth, because that tells me how off on a tangent the node is likely to be.
the node depth is not a consistent measure of subject relevance. occasionally, replies between monks approach conversation level, and the depth increases dramatically. this does not mean they have moved off topic.
for instance, i've seen nits worked out of algorithms presented previously, and not patched in the nodes above. if you're unfamiliar with the subject of the thread, and the authors involved in the discussion, it's difficult to generalize relevancy from node depth alone. so perhaps instead of writing that the depth tells you the relevancy, it would be more accurate to say it is one measure of relevancy. no?