It really depends on the focus of the game. Games like Team Fortress 2, Fat Princess, Street Fighter, Battlefield, SOCOM or Starhawk are multiplayer based games. Single player mode is mostly added as a bonus for such game and no one buying it (that actually research it) should feel ripoff. Starhawk is the sequel to a game that didn't even had single player. If that is the kind of game that interest you, if single player mode is nothing but an big tutorial of the features, weapons and vehicles used in multiplayer, then yes, its excusable.
On the other side, we have games like MGS 4, GTA 4, Bioshock 2, Darkness 2 or Red Dead Redemption. Do you think it would be excusable for them to have a tackled on multiplayer? Would it be ok for them to be punished by it? Would you agree with a reviewer that gave Metal Gear Solid 4 a poor score because the multiplayer was lackluster?
On the other side, we have games like MGS 4, GTA 4, Bioshock 2, Darkness 2 or Red Dead Redemption. Do you think it would be excusable for them to have a tackled on multiplayer? Would it be ok for them to be punished by it? Would you agree with a reviewer that gave Metal Gear Solid 4 a poor score because the multiplayer was lackluster?