Multiplayer is fine and good, but just because you can include it doesn't mean you should.
I know that to a certain extent I've commented on this issue before. When I previously talked about multiplayer using Uncharted 3 as a focus I argued that there would be nothing inherently wrong with splitting the single and multiplayer components into separate releases then charging less for each component (roughly half price, so that buying both would cost the same as a regular title). That way if a person didn't want multiplayer they didn't have to have it or vice versa, and also allows the publisher and developer to gauge the interest and potential merit of each part of the game experience. Uncharted 3 might have been a somewhat unique -- although becoming less and less so -- case, but now I'm going to talk about something else entirely: I'm going to talk about cases where games don't need multiplayer at all, but where it's being shoehorned in anyways.
The need to not include multiplayer is a tricky one, after all there's no telling sometimes what will and won't work. I would have been extremely skeptical of the Assassin's Creed multiplayer, but Ubisoft actually developed it into a comprehensive and from the looks of it extremely fun and full experience. What I'm generally talking about though, are games like Dead Space 2 that have a fairly mundaine or even subpar multiplayer component tacked on just so that the marketing company can scream "Look! It has multiplayer!" at you in hopes that it will make you buy it.
In my mind that seems to be the only real reason why games like Ninja Gaiden 3 are getting multiplayer additions. It certainly doesn't look like the worst thing in the world, but does the inclusion of a multiplayer element in what until now has been a solid single player series make me stand up and proclaim "Now I must buy this!"? The answer is no. I might get Ninja Gaiden 3, but if I do it's certainly going to be for the same reason I bought games like Devil May Cry, Assassin's Creed, No More Heroes, and others: it's going to be for the strength of the single player campaign.
I can see where the temptation to shove a multiplayer option into games comes from. After all you can increase the longevity of a title significantly if you give the player a reason to keep coming back, and multiplayer in these cases serves to add a level of randomization to the mix: the promise that there will always be different players, more matches, etc. You can find your niche, strive to make a name for yourself, or just do a match or two in your spare time to burn off some stress.
The thing is though, even with multiplayer it's never going to last forever. People will get bored, or something newer and shinier will demand their attention whether it be the sequel or some completely different beast. I know that some online communities like TF2 are still going strong years after the fact, but look how much work has been put into that game, and then remember that TF2 is only multiplayer, there is no single player mode to engage in unless you have some twisted obsession with kicking the asses of computerized opponents.
I know that players want to feel justified in their purchases, and that including multiplayer is perhaps the easiest way to say "look, hours more content", but it's not always the best way: Skyrim proved that last year. Would Skyrim have been definitively better with a multiplayer component? I really doubt it. Instead it would have only pulled resources away from the main game, which in turn would have diluted the single player. I can't speak to whether it would have been a completely different game, but my point is that it didn't need to be. Skyrim was and is fine standing on the strength of how it presents itself in the one person experience, and I honestly wish that more games would strive for that sort of presentation these days instead of just figuring that letting you go online is a cure-all to be bleed dry.