Only advantage of a single player mode is to hook the new people (who've never even thought of piloting a 'mech, or BA suit), it'd be great to have it but not necessary to the mods short term success I think (although I really hope they do).
I wouldn't say that. Some of the best games I have ever played were more or less exclusivly single player (Deus Ex, Braid, Fallout 3, Morrowind, Shadow of the Collosus, Metal Gear Solid, etc).
Single player also has the added benefit of creating a memorable impactful experience AS WELL AS fun. Something that's there where you can truely immerse yourself in whatever role.
Imagine... MWLL, with all the Mechs fighting about, vehicles and the aerospace causing a complete war... and imagine you the player as a battlesuit thrown in the middle of this chaos. That, is an experience that if crafted right can transcend anything that can happen in multiplayer.
Sure these moments can happen, and they will in multiplayer, but there's a little bit of a different kind of extra "magic" that happens to you, the player, when you have those scenarios happen in a controlled randomness, through the venue of single-player. For example, singleplayer is like attending a wonderful orchestra, it's a planned experience that can give you a mix of emotions and be powerful as well as impactful if orchestrated with perfection. Multiplayer would be like taking a group of all your friends, and making your own orchestra; there is a lot of fun to be had and you have the added benefit of experiencing a dynamic experience that can never get old, and through this you might occasionally orchestrate your own little bits of charm and awe, but it's not the same thing as attending a professional performance.
That said I'd love to map some single-player stuff eventually. It's just a matter of having enough time, and if we can get it to work or not.