And? How would that game not benefit from having simple multiplayer? With a few simple systems (chat, player list, players walking through each other, maybe a scoreboard, it already has medals), that game would maintain its current state, while building a community, advertising itself better, and allowing people to enjoy it together, somewhat. If they wanted to take it to the next level, they could make it so players could party together and interact in other ways. Lazy.
NEStalgia? In any case, I feel this argument could go on for ages - and I think you realized that there are two sides to this with my AI argument.
I think it's safe to say we can both agree that at least when it comes to BYOND (though for most online-supported consoles as well), a safe mixture of both single and multi-player should be in order. Again, I think Minecraft is one of the best examples of it (give the player the choice), but games such as Starcraft, Halo, Call of Duty, etc. at least implement a campaign. Probably one of the finest examples of seamless integration was as you said, Left 4 Dead, or for BYOND, Casual Quest.
But yes, I agree, the best games should have seamless integration of both single and multi-player so the client gets to choose how they interact with the game.
And? How would that game not benefit from having simple multiplayer? With a few simple systems (chat, player list, players walking through each other, maybe a scoreboard, it already has medals), that game would maintain its current state, while building a community, advertising itself better, and allowing people to enjoy it together, somewhat. If they wanted to take it to the next level, they could make it so players could party together and interact in other ways. Lazy.
Just because your game is multiplayer doesn't mean it shouldn't have AI. In fact, I'd say the best multiplayer games are the ones that are AI driven with multiplayer co-op. WoW, L4D, Army of 2, Red Dead Redemption, even the newest CoD games have multiplayer modes that you can play against AI - though they are primarily PvP. A game being multiplayer only/not having AI is just about as lazy as one being single player only, sometimes more-so.