OpenBSD's installer is pretty good, but I find the docs for partitioning are written for someone smarter than me. The old docs made it easy, so maybe I've just gotten dumber.
However, (this may be unique to OpenBSD, you can't do it with Free and I've not used Net or Dragonfly in a long time) if you're multibooting, like Linux, it can use a logical drive and doesn't require a primary partition. It works pretty well on a lot of thinkpads as its developers seem to use those a lot. I have a page on multibooting it--last time I looked the FAQ said it's complex and such, but I find it quite straightforward, at least on a Linux machine.
Going back into the reasoning--lots of times, folks just want a BSD on their laptop. There doesn't have to be a reason. I remember this great statement, which at present I can only paraphrase. It was when alpine, the mail client, was called pine and it was one of the many arguments about pine vs. mutt. The author said, People pull out all sorts of technical reasons to justify what is, in the end, an emotional decision. So if someone asks me why I want a BSD on my laptop, I feel justified saying because I want to. (Which makes me think I should put a link to the video that, I think, first got Billie Piper, Rose in Doctor Who, known in England. It was a song called Because we want to or something similar. With the confidence of youth it has lines like Why ya gotta play that song so loud, Because we want to.