Puppies and Plots

• Throwing in a little pooch or kitty can make an old character type feel new and fresh again. Seems simple enough.
• Cat people are stereotyped to be crazy and moody but dog people are thought of as social and cooperative. Utilizing this concept and manipulating it to surprise people can be a useful strategy.
• Even if the characters don't have a pat currently, implying or stating that they may have once had a pet or currently want a pet can do just as much for a story.

 Thoughts:

• I don't think that it is this simple. The pets need to have a purpose and if they can be easily replaced or removed without much change to the story, then they shouldn't be included. The closest example to this I can think of would be Mia's pet cat, Fat Louie, from The Princess Diaries. He doesn't serve much of a purpose in the movie other than act as a companion to Mia (which is arguably an important role, but probably not as important as Toto from The Wizard of Oz) 
• The "dog person" "cat person" stereotype isn't completely false, but it isn't completely true either. Cats are good pets for people who can't get up and out as easily where a dog is a pet that needs a person who has enough energy to get up and take them out every now and again. However, a person could be very social and have a little friend back at home who doesn't require to much maintenance and someone who is introverted and antisocial could have a dog to keep them company or provide a sense of safety. It all depends on the person.

Question:
 In what situation would a pet be detrimental to a film? Perhaps it wouldn't make sense to have one or it would take away from the plot.