I certainly don't blame Douglas Adams; at the time none of the other "completions" had been done, and why not retool a story that hadn't gotten to see the light of day?
But it was weird that he also threw in so much of the plot of "City of Death," a story that definitely did see the light of day and is one of the better-remembered Tom Baker stories.
Eh, as Terry Jones said about him, he loved ideas but hated to write. It's the reason his Doctor Who stories (presumably including Shada
) never got novelized before now-- he wouldn't give permission for others to do it because he wanted to do it himself, but he had to be literally locked into a hotel room to actually write his books, so the novelizations never got done. Heck, even the Hitchhikker's Guide was a case of him recycling the same story from medium to medium. So I could easily see Dirk Gently
being his way of redressing his Doctor Who work in a way that spring-boarded his imagination into creating a new universe to write about.