Spike was created because he was a holdover element from earlier pony iterations, Faust was only being thorough, but Spike as a character in FiM serves to act as a little brother surrogate for the little girls intended to watch the show. Little girls are surprisingly prone to nurturing at a weirdly young age, baby dolls is an example of this. So it isn't farfetched to realize even these little girls want to experience a type of power fantasy, where they (the ponies) get to lord over and dominate over the stupid ugly gross boys that usually take precedent in real life. Spike, Snips and Snails, maybe even Big Mac, were placed in the show to be styled on, corrected by the girl ponies, because this made the girl viewers happy. In the beginning Spike was a sarcastic jester with his background obscured because little girls didn't care and he was supposed to be ambigously ignored so the girl gang could get their tea parties on. Only during times course Spike got somewhat more integrated because the writers realized they were writing for unhinged freaks and not little girls, they attempted to make him fit in more, realized it didn't work, and so he remained the shittest.