The single player mode has been suffering at the expense of multiplayer. I personally find this to be a problem with some of the lesser known RTS games these days as I'm not really a social gamer and I typically play single player. Without campaign mode there I am stuck playing random matches against CPU opponents and that increases the learning curve dramatically. The single player game can be seen as a sort of extended tutorial. You start with very few options at your disposal and as the campaign progresses you unlock more and more things to add to your knowledge base. By jumping strait into free battle you have access to all the capabilities of the game from the start, which can provide an overload of options.
Homeworld is a good example of doing this right. They start you off with a quick tutorial level. They teach you how to move units. They teach you how combat works. Add a little basic strategy into the mix in the form of formations and you have the general idea of the game down. Since the tutorial was done with fast moving but weak fighter units you get an idea of how maneuverability is handled in the game and you learn how to work with one unit type.
As the game progresses you gain access to more unit types and you learn practical applications for them. In the first level after Homeworld's tutorial you gain access to corvette class ships and if you use them in combat you get to see how much the heavy corvette shines in combat against higher maneuverability opponents. And later on in the game you face the swarmers and are brought to face the strategy of resource disruption and the fact that these extremely high maneuverability and high firepower fighters have one glaring weakness.
Outside of the tutorial aspect, single player allows for experiences completely impossible in multiplayer. AC:VD was the closest of my knowledge to be able to pull off single-player-like battles in multiplayer with its unknown weapon fights, which would be considered co-op only boss fights. However, having something like that controlled by another player is out of the question. (Although, if someone were to pull it off properly I would not have a complaint about its existance.) Mostly because of the sheer number of attacks at its disposal coupled with the totally non-standard control scheme that would be required to move something like N-WGIX/v.
Being able to break from the typical multiplayer experience where everyone is on a more or less even playing field is part of what adds to a game's shelf life in my opinion. If a developer focuses completely on multiplayer there the game can get dry very quickly, especially for loners like myself, because there is very little variance in how online matches go.
However, I don't mean to say that every game should have single player. Just as adding multiplayer just for the sake of multiplayer makes for a very dull experience, so does adding single player just for the purpose of single player. Sometimes you just have to pick one. Taking League of Legends as an example: if they had decided to add a single player mode, the quality of multiplayer may have suffered. That isn't to say that its multiplayer is perfect, but given that it's all they are thinking about they can put their full effort into it and they won't have to divert effort into building a game mode that may or may not add anything to the game itself.
The same can be said for pretty much any fighting style game on the market. Street Fighter, Tekken, Guilty Gear, etc.. While they may have story, the single player mode is an afterthought and it shows. After all these types of games are for Vs mode, to show which of two players is the best. I'm not saying that story should be completely neglected, as a well painted backdrop is essential for having an ideal experience, but it should be considered how much single player can add to the game as a whole.
In these types of games the pacing differences of story vs fighting makes for rather jarring transitions.
In a similar manner as with Homeworld, Dead or Alive seems to be taking the right approach. Lately they have been using story mode as a sort of extended tutorial. Most missions start with them showcasing a technique of some sort, and then setting up a mission having to deal with that technique. It helps a lot for players like myself who have just started the game. I'm also starting to see some good characterization in their cast.
However, I will make sure to say that single player is no match for playing other people in terms of getting high level play. In order to challenge high level players the CPU usually has to resort to input reading, which can easily be manipulated. As I have found in Soul Calibur 3's tournament mode, the CPU simply can't handle repeated 22/88b from Nightmare.
There are pros and cons to creating a single player mode. However, being the introvert that I am, I would like to see more games putting effort into it.