Let's say that you're an irc network operator trying to make the channel's entire history searchable (e.g. botbot.me type thing).

For absolute correctness, you'd want to record per-leaf and merge.

However, the last time I needed log-merging functionality was much more boring; some of my log-files were corrupted, and someone else had logfiles that weren't as extensive, thus I needed to munge the two together. The timings were subtly different (because that's how irc works), so I wrote custom code to munge them together, preferring his for the range of corruption.

So yes, I've needed that functionality, though it wasn't actually done due to a netsplit. I can foresee it being useful in the case of a netsplit if your logging is at the ircd level or you run one bot per leaf.