As long as it doesn't take away from the single player I see no reason why a MP mode of sorts shouldn't be included in every single title.

Resident Evil 5 time! I hated the fact the series switched to a coop focus. Sure the game technically could be played SP, but it was really designed for MP. If the game had a great single player campaign like RE4 and a separate optional coop campaign that doesn't involve the main characters (so you see the game from a different perspective) I may have enjoyed the overall game more.

MP isn't bad, but poor use of it is. Chaos Theory is a brilliant SP game... It had an optional MP mode. Nobody ever complains about MP in that game. At least it's there and in way takes away from the SP experience.

If you're going to add MP to a 3rd installment of a series, you might as well just show the finger to your fans. Even the 2nd installment of some games does this.

Ground up with MP, depends on the game itself, and the effort; however tacked on MP? No thanks.

Uncharted 2 in my opinion is one of the later games released with a proper way to add multiplayer, and it was correctly done. Older games like Socom 2 and Goldeneye not only set the standard for a lot of games like it, also proved if you make a game good with both SP and MP fans will continue to support and play for years (yes Socom 2 can be played over LAN).

Many singleplayer games use AI players as part of your team or enemy and I see no reason why they couldn't be replaced with real players to add variation.
I love playing Co-op with some of my favourite experiences being with Command & Conquer and Quake games. I can't think of 1 singleplayer game where I wouldn't have also enjoyed playing it in co-op with a friend too.
So, should all games be multiplayer? well it rather depends on the multiplayer format they're referring too.

FUCK NO, all games MP what about gamers like me who want to play single player or 4 player RPG's like the Tales games which you can play with friends together.

If you want MP only games you no offense are a BLIND IDIOT. That means that when the MP goes offline there goes your WHOLE game. This is how the industry can FUCK OVER their fans. This would be the reason I wouldn't buy any Next-Gen consoles.

Bioshock Infinite would've been delayed (again) or have its single-player aspect suffer. Two things I don't people want. And I'm sure being only single-player doesn't take anything away from it

Adding Multiplayer usually takes away from the SP or delays the game, maybe even both (or none if possible).

Depends.. like Tomb Raider, Uncharted and the like focus should be put more on single player.

Halo, Gears of War most likely 85% single 15% multi-player.

Racing games about 50% each.