Because I thought if ever Rory were to seek advice about love in the TARDIS, Ben Jackson would be the best person to get it from. And because Methrel was right: there's not enough Ben/Polly out there.
Disclaimer: I own none of these, I just love them.
Disclaimer II: I know that as far as we saw on TV, the Doctor didn't mention being a Time Lord while Ben and Polly were around, but I figure he had to have explained himself to them at some point.