Sean Gaffney pontificates:

Because Ataru actually loves Lum?  Because, as Kun-chan said, Ranma
wouldn't save Shampoo if he could save Akane, and Ataru would probably
not save his other "girls" if he could save Lum.

        OTOH, Ataru wouldn't save Lum until he could find some other reason to 
be in the right place at the right time :-).  (Look at that story where he and 
Ten rush off to save Lum from a kidnapping, and Ataru pretends that he's just 
along for the ride, that he's along to get a chance at Ran, etc, etc)

 (Actually, Ataru would
probably figure out a way that they wouldn't be in danger in the first
place.  I've been reading more UY lately, and feel that, especially
towards the end of the series, Ataru's intelligence is dangerously
underestimated.)

        I don't see this.  I've never been of the "Ataru is dummy" school of 
thought-- after all, he is several times shown to be as smart as Mendou-- but 
he's no genius.  He is davka (can't think of a good English word for this, 
probably "contrary") to the point of getting them into trouble, his schemes tend
to be half-baked, and so on.

 Remember, Shampoo was Rumiko's attempt at doing Lum
correctly.  IMO, it worked:  Shampoo is far more annoying than Lum.

Where did you read this?  I've never thought of any connection between the two 
until this thread started.  (I'm also that relative rarity--a UY fan who doesn't
find Ranma very interesting, so I might be missing something)

Gavin