hoping it would have a lot of replay ability
read: "We don't like people buying pre-owned copies, so we thought if we just threw some MP in, some people might wait a little longer before trading in, in which time we might have sold more brand new copies. Actually... Hey! Hold the press, guys! Let's put some really awful and time-consuming MP achievements in. Some of the more OCD gamers might hold onto it even longer if we insist on putting a 'get to level 50' achievement in as well! That'll keep pre-owned off the shelf for a bit!"
Co-op, when done well, is often a good addition - Dead Space 3, by all accounts, is an exception. Competitive MP is, in a lot of games, completely unnecessary, and added in for the reasons outlined above, rather than to give the consumer what they actually want. Anyone who says otherwise is, IMO, misguided or lying their **** off.