I'm sure there's a genuine reason, but since living near Heathrow for the last 2 years, I've seen over half a dozen A319/20/21's way out over Windsor way, climbing away from Heathrow with gear extended
Its standard procedure when one of the brake tacheometers is u/s. The gear is left down for one minute after departure to allow it to stop spinning (the normal automatic brake application before retraction doesn't work) before you raise it. Flying around with the gear stuck down is actually no big deal unless the air/ground logic thinks you're still on the ground at which point it gets a bit more interesting with no AP, A/THR or FDs! Raw data flying practice when you least want it!