If there was enough spike centric episodes to of set that kind of thing, sure. Or a comic or whatever, sure I would be down. But their isn't they just exclude, him. Why? They don't give a shit about spike at all, I don't even think they like spike. They even made him a dog in that dumb ass movie, not a child you know something that can be viewed as some type of equal or a better considering he is dragon, or considering the fact everyone treats spike like an person in the regular show . No, a dog.
There is no issue with writing or having him there, they don't like and/or care about spike.