My WRX is the third Subaru I've owned (including a '98 and a '99 RS). I downshift into first at maybe 10MPH on a regular basis. The car has over 12,000mi on it, and never a problem. Yes, 1st gear is important here in Seattle. I've done this on all of my Sube's and never once had an issue... I can downshift into 1st at 20MPH after double-clutching, and it slides into gear easily.
I do no abuse my transmission -- I never do a drop-clutch launch; I will always ride the clutch slightly in first so as to save the gears (besides, building boost is important).
Rule #1. NEVER FORCE THE CAR INTO GEAR.
Rule #2. DON'T DROP THE CLUTCH.
Rule #3. IF YOU IGNORE THE ABOVE, DON'T B17CH WHEN YOU HAVE A PROBLEM.
If you have a problem and you're treating the car right, well then, that's an issue. Unfortunately, when dealing with complex mechanicals, one must first rule out user error.