RtCW will have multiplayer out of the box. Otherwise, it will never have multiplayer. The game's underlying application won't stand for a patch that enables multiplayer. Somehow, they'll figure it out.
Network play is one of the most important factors in a best-selling game these days. Id Software isn't a company to leave that out of its releases. Quake III is nothing without multiplayer; the single-player campaign was worthless.