Scrappy Doo is one of the most divisive fictional characters in animation history. A lot of this depends on your age. If you were old enough to have something invested in the various Scooby Doo cartoons by the time his nephew, Scrappy, was introduced in 1979 then you might be more than a little resentful of the character. If you were very young and new to the whole idea of Saturday Morning Cartoons, then you probably have some fondness for the pup. The same rule applies for Jar Jar Binks, oddly enough.
Mark Evanier wrote for Hanna-Barbera animation when Scrappy Doo was added to the cast. He's posted a very detailed story about the character's creation and thinks Scrappy Doo played a significant role in keeping Scooby Doo on the air when it was in danger of getting axed. Here's a sample:
"He did not destroy the show, as others claim. He was born during a time at that studio when characters were treated a bit too much as a commodity and in that context, he served his purpose. And every now and then, I come across someone — usually someone a lot younger than I am — who absolutely loves Scrappy to pieces. That's great. I don't share the sentiment but I'm glad to have had a hand in giving those folks something they like."
Read the five-part story at his website News from ME. It's one of my favorite sites to visit, btw.