Almost nobody ever pairs Ranma up with Shampoo because he doesn't love
her.  First of all, when she was introduced she was out to kill him
(Collected Manga volume 3) Second, (in the same manga) he doesn't have any
attraction to her (Akane is the one to point out that she's cute).  When
she is re-introduced (Collected Manga volume 4) she's cursed to be a cat
(a definite minus in Ranma's book) and throughout the series (as far as
I've read) he NEVER gives her ANY encouragement.  Akane he insults, but he
always rescues her and in the last Manga he admits his love for her.
Ukyo's his best friend and he calls her cute.  Kodachi...well let's leave
her out of this.  But with Shampoo the only things he likes about her is
exactly what you said 1) she's attractive (he does mention this in the end
of volume 3 of the manga) 2) she can cook.  Unfortunately he has NO
emotional attachment to her the way that he has with his other fiancees
(excluding kodachi of course).  If it were to come down to it Ranma would
probably choose Akane first, if not Akane then Ukyo, if not Ukyo, Shampoo.
You're right that Shampoo isn't the worst fiancee by a long shot (that
award goes to Kodachi) and you're right that Shampoo probably does care
about Ranma.  But Ranma DOESN'T care about Shampoo that way.  Which is why
so few fanfic writers ever pair them up.  
Thanx for litening to me rant and rave.

which is why it would be nice to see some fanfics that attempt to pair the
two of them! What would it take for Ranma to begin to feel something for
Shampoo? it's a difficult task to get the two together, but one I vow to
conquer in my next fanfic!!

Ryan