I think the issue of In Battle pages was that they were a relic of our pre-fork days before we split off battle forms (like Mettaton EX) to separate pages. I think the idea was that we could shove everything about any battles onto those pages, and it spread out to other boss fights.
The problem with merging them back together, however, is that the resulting page would potentially be extremely long to read comfortably. This won't be a problem for pages like Napstablook or Mad Dummy, but it will become very apparent on pages such as Sans and Sans/In battle, both of which are very long pages in their own right. There isn't an easy solution to this problem outside of picking and choosing which characters actually deserve subpages.
We may consider making subpages for characters with complex and/or multiple battles, such as Rouxls Kaard and Knight. This will, of course, coincide with the decision to merge some older in-battle pages and may not happen until things slow down here.
I'm actually working on a potential solution to a similar problem: group fights being spread across separate articles covering the same battle. You can see an example of this here, where I cover the group battle against Susie and Lancer on a separate page. You can read more about this proposal on this post, though I do plan on making a full post at some point when I'm not busy with other things.