Easy (co-)oportinity

When Gears of war came out with online co-op it seemed to be a must for games to have. Many games have jumped on the bandwagon such as Halo 3 and COD: world at war. With co-op came a social experience, while still progressing through the story of the game. However, compensation for having multiple people seemed to be the opposite of what games needed. Co-op created an enviroment for even the less skilled gamer to pass a campaign on the hardest difficulty with the help of a few friends. I'm not saying that this is a bad idea in fact it makes a gaming community more tight knit and makes what was once a solitary activity social. But, with the new player came no difficulty modifiers and thus made games easier. Why is it that when a game becomes co-op it has to be easier? It seems to me that with each new person new enemy's could be added or the enemies could be harder to kill forcing the players to work together. Instead, the enemies are hardly more difficult (if even that) and the number doesn't increase. This makes the game easier because twice as many bullets are flying (four times as many in Halo 3) and it even becomes harder to die. For example, in Gears of War a player has to restart from the last checkpoint in single player but can be brought back to life without penalty in Co-op mode. Taking skill away from games can make the games dull and boring no matter how social the experience is. Despite my complaints, I find myself wanting more games with co-op campaign as long as the difficulty is maintained. Left 4 Dead is a great example of how to use co-op. Having a human take over for the AI makes the game slightly easier but keeps it difficult because even if the AI can't shoot as well as your friends, they can certainly distract the enemies. No dynamics of the game are changed while playing co-op as well. A person is still killed and revived the sames, way. The only noticable change in gameplay is that you can talk to your partners. I wish more companies took the example of Left 4 Dead and kept a difficult game difficult and constant whether a player is playing co-op or single player.
I would like to take this chance to try and minimize criticizm on my difficulty statements, yes video games have been getting easier over the years and i wish this trend would stop.