I've got my own headcanon for Ford (and Zaphod, as it follows) for why he brought Arthur along, which also explains why he doesn't seem to think it's a very big deal that the Earth is no more. The basic gist of it is that what with Betelgeuse being on the verge of supernova as it is, and from our perspective could actually go at any time, it already has. A red giant goes supernova? It's not just going to take out its own planetary system, but several nearby star systems as well. That Gretchentown, New Betel that Ford mentions to Hotblack, in my headcanon, is actually a totally different planet orbiting a totally different star just north of Orion's Belt.

I'm sort of starting to explore these reasons in my Nano fic that is taking way too long to finish, but I do plan on doing something a bit more with it later.