Backing in is often easier, if the slip is upwind and your bow tends to get blown downwind (often the case for me). The other nice thing about backing in, is that, as opposed to backing out, you tend to have steerageway earlier (again, on my boat: backing out is pretty nerve-wracking since I often can't steer until I'm already out of the slip, and there's not much room to turn after that).
Still, that doesn't explain why they would "strongly encourage" it. What sort of dock is it, and what town (if you don't mind my prying)? In some berths I've seen, you get pylons and are expected to use a Mediterranean moor. How do you tie up?
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27