Yes, I've already spoken at length about how the exclusion of multiplayer makes very little sense when it comes to Assassin's Creed: Syndicate, but there's more to it than that. The game is just basically begging for a multiplayer of some kind, because of its entire meta-game in the story.
The Assassins and Templars are both running gangs, who are even color-coded. You face off against rival gang leaders in the main story, leading brawls against their goons and finally fighting them one-on-one. Meanwhile, your faction accrues power and influence, gaining new gear and resources. That doesn't sound like a single-player game, that sounds almost like one of those Facebook "strategy" games.
Now I am not saying Assassin's Creed: Syndicate should emulate Facebook -- far from it (please Ubisoft, of all things, do not emulate Facebook). All of the traditional multiplayer modes in past games make way more sense here. Almost every mode makes sense in these circumstances. Wanted, Assassinate, Manhunt, Artifacts, and Wolfpack in particular make way more sense. You need to steal from rival gangs and take out opposing gang leaders. A simpler set-up could not be handed to Ubisoft.
The multiplayer could be heavily integrated with the single-player, and better yet, they could finally make offline variants. Imagine having all the content of past multiplayer titles, but you could also play it against bots in-game. It could even increase mission variety. This is so obvious that it just makes the exclusion all the more disappointing.