Well, 
I personally think that it is a storytelling 
device to make us as the viewer more sympathetic 
toward Ryoko. To show her vulnerable side endears 
her to us. It's hard to actually hate Washuu-chan, 
no matter how badly she behaves. Being in the form 
of a child is another storytelling device to allow 
her character to do things that would normally 
make the viewer despise a character, but she does 
it in such a cute, childlike, playful way that you 
are more inclined to forgive her. Also, the 
juxtaposition of the adult nature of Washuu and 
her childlike form makes her a more complex and 
interesting character. Likewise, showing Ryoko's 
vulnerable side to the audience deepens our 
appreciation of her. It lets us know that the 
rude, vulgar and obnoxious things she does cover a 
very tender side that she is terribly afraid to 
show. 
It is all part of the wonderfully complex 
storytelling that makes Tenchi a really great 
story. Ryoko's combination of aggression and 
vulnerability is incredibly appealing and is why 
the character (and cels of her, of course) are so 
popular.
Now, I wonder how much that pan cel of Ryoko in 
'the device' would be. Guesses, anyone? That's one 
Ryoko cel I'd pay upwards of $1K for.
Ryo