Here's the gist of the piece: Imagination has survival value because it allows one to choose the best plan by running through possible consequences. This is meant to be a response to the (imaginary) interlocutors who "downplay the cognitive role of the imagination" by restricting its role to discovery rather than justification. Rather: "... imagination is not only about fiction: it is integral to our painful progress in separating fiction from fact." The author is a logician, and is apparently is unaware of the self-parody involved in his assumption that if imagination were involved only in discovery and fiction, it would somehow be debased. But he's also worried about justifying the ways of philosophy to science, or at least "Critics of contemporary philosophy," who (remaining unnamed but amply imagined) claim that philosophy "loses touch with reality." Self-flagellation before and alms-begging from the scientific royals (note the use of the word "painful" in opposition to "playful").
There's the further irony that the kind of (painful) caution that makes this piece -- as one commenter notes -- an exercise in "stating the obvious" is just as bad for the reputation of philosophy as airy (and imaginative) speculation (read the other scornful comments as well).
Ahh, ye self-hating philosophers: grow some balls (figuratively, in your imagination) and stop apologizing. You're damned either way, so you might as well have the courage of your convictions.