Wow - thanks to all who answered, especially Craig and Andy. I
understand now why weighted wing tips are good.

I’m puzzled about Andreas’ comments on the ASW-22, though:

Even with completely filled outer wing bags (about 120 lbs per wing)
the roll rate doesn't suffer at all.
But filling the inner wing tanks (about 100 lbs per wing) instead of
having a co-pilot on board reduces the roll rate significantly.

This behavior seems to defy physics!

-John