Imagine a U shape for the socket and water getting into it without a drain hole at the bottom. The socket will eventually fill over time to the point were the fuse would blow.
I doubt the fuse would ever blow for rain water. It'd need to flow around 20 amps, if you've ever played around with a HHO generator you'd appreciate that you need to have a hell of a lot of surface area, not to mention chemicals in the water to even approach that current at 12vDC.
The worst that is going to happen is the contacts will corrode and the outlet will stop making good contact.
Use the power output as necessary regardless of the weather.
If it doesn't have a drain hole then make sure it's dry when you put the bike away.
Coat the internals with protective grease (silicone grease, vaseline, ACF etc). Then remember that said grease will be on your devices' plug after use so wipe that or afterwards or you'll be leaving grease on everything it touches.