Online multiplayer is becoming an increasingly important factor in my decision to purchase games. While that may just be me, it appears that people overall like to play games with other people. Social media games are a good indicator of this, though, I don't really consider those multiplayer for the most part. They do indicate that people like being able to at least partially, share their gaming experience with each other.
When I'm looking for a new game, I'm interested in it being online multiplayer so I can play with my friends. I don't have friends over for LAN parties so much these days, so this is the best way I can connect with them. As the age of the average gamer increases (The average age of the most frequent game purchaser is 35 years old.), the likelihood of having friends over all the time (like we did back in our school days) decreases (or maybe that's just me again?). Online multiplayer allows us to stay connected. I'm all for sitting down and playing a single player game, but it's even better when I can invite a friend to play too.
It's not as if people won't buy your game because it has an online multiplayer option. If they're looking for the type of game you're making, and they're interested in your game, the multiplayer over the internet is not going to dissuade them.
Additionally, multiplayer over the internet means increased sales for you. Now instead of two people playing one copy of the game, you have two people playing two copies of the game together.
All in all, I think adding online multiplayer to a game is more likely to increase sales than adding any other single feature. With it's ability to double your sales for people buying together, and the option of adding bundle deals for multiple copies.
But is it worth it? That's up to you to decide. Perhaps for now you can keep online multiplayer in mind, and design as if it's there, but save it for later.
An article related to this topic: The Importance of Multiplayer Gaming
And the importance of local multiplayer too!: The importance of split-screen, co-op and basically all offline multiplayer games