I think it depends on the people...the reason we got Layla in the first place was because of our other dog. We had a VERY aggressive dog (we joked around she was a serial killer...she was THAT aggressive...) Long story short, we had done some research and read some articles that had stated one of the best things for an aggressive dog was to actually get them a puppy. Needless to say, we were thrilled to have an excuse to get another dog. Its not that we were only getting it for her, but any excuse I could use to get another dog, I'd go for. It did make Olivia better, they fell in love instantly...slept in the same crate, chewed on the same bones...they were adorable. It didn't help with her human aggression issues, but she was amazing with this little puppy.
I'd judge it on a case by case basis. If someone comes looking for a dog to take care of their old dog so they dont' have to, that's clearly not a good reason. If they're looking for a companion, I think that's great. In our case, we wanted another dog and she served a purpose. It sounds bad to say that Layla came to us "for a purpose," but we were trying to save Olivia. In the end, she couldn't be saved
...but the time they did have together was much better than before we got Layla.
Overall, I think you have to judge it on an individual basis. You can't rule out each case just because they want a dog simply because they want a companion for their previous dog, but if they want another dog so they can ignore their dog at home, that's certainly not a valid reason to put another dog in their home.