Ghosts'n Goblins was Capcom's first big multi-million-selling series, and Arthur was even referred to as "the face of Capcom in the '80s" on the official MvC1 site. He deserves to be in a Capcom Vs. game.
I wouldn't remove anyone from the roster. Individually, I like each character that was included. It's just that 30 slots isn't enough -- the base roster should have at least equaled MvC3's.
I agree, though I wish Spencer was his OG Spencer and Firebrand was Demon Crest.