To be honest, I understand why Alucard was made a boss rush-only character. Efrain already plays more like Alucard here, so having Alucard as well in the main game would feel redundant and amount to not much more than a spriteswap with some voice changes. Alternate characters in other Castlevania games have at least offered something different gameplay-wise, so Alucard wouldn't be sufficient as-is.
EDIT: Just realized Alucard would probably have some exclusive abilities, but that's about the extent of that, no radical diferrences.