Firstly, (and I say this every time!) not every game needs a multiplayer mode. More often than not, games that have the most successful multiplayer modes are the ones that people are happy to buy and then ignore the single player modes available, i.e. Gears, CoD, Battlefield and Halo - there are many, many people who play these games endlessly without every straying into the campaign.
Secondly, when a game is reviewed, the whole package is taken into account, so if the developers put in an unoriginal or sub-par multiplayer mode, then it will affect the reviews and so potentially affect sales, and for a game such as Tomb Raider, where it is all about the story, this could be particularly problematic.
Finally, I’m afraid we’re pretty much at the point where there are very few games that can actually compete in the multiplayer world with the big 4. It is also the case that any game attempting a multiplayer mode is compared to that big 4, each of which has had years to hone the quality, which in turn means that they are effectively setting themselves up to fail.
The multiplayer mode in Tomb Raider will have a small spike in play upon release as people have a look and chase the inevitable achievements (which will have to be included in order to entice people to play the mode for any length of time), but then it will die a death just as in every other game that has attempted (and failed) to compete in what is a very closed market.