I think it would be kind of surprising if you didn't see it coming! Generally, if a forced sequence leads to checkmate and the other players sees it, he'll resign. Some lesser players (like me I suppose) would like to analyse and understand each forced move, and see what lead them to that.
Though having a setting like this could be a nice idea; another version of the same idea would be to have something saying "Forced sequence to checkmate: Do you want to resign?"
I think the idea is good and could be used, though if it doesn't lead to checkmate, conditional moves are doing great; you can just wait for the player to get to the end of the sequence. Well, that's only my incomplete point of view!
Idea for the site: How about a setting to automate your move if you have only one legal option? Legality checking is already in place, because you cannot drag a piece to a square it cannot move to, and so it would simply be a case of if the setting is enabled for that player, and the list of their legal moves only one move long, automatically submitting that move on behalf of the player, much like a conditional move.
If a player so chose, this would speed up certain games, especially those involving longer forced sequences.
Does anybody else think this would be a good addition to the site?