Using E:WoM as a basis for how used the multiplayer component is/was used is ridiculous. It was - unfortunately - a terrible game; one that I was embarrassed to have followed so avidly. As such, I never really touched the MP component. I doubt I would be alone in feeling that way, either.
The Civ games and Age of Wonders (sometimes even PBeM, but many live games too) have lived much, much longer in my 'current' gaming rotation over a span of many years due to their ability to be played MP. MP can make a good game great and a great game better still, but it can't magically fix an awful game into being anything other.
All this said, FE is shaping up to be pretty spectacular so far. It is beginning to reach the potential of the first and it would be an absolute crying shame if MP was neglected. No doubt I'd play it anyway -- I certainly have been so far -- but I can guarantee that it would not have as much longevity without.
Who cares? They have your money already? Well, yes, they do -- sort of, I preordered WoM so they don't have any additional money from me at this point -- but I can say that other friends and even just online acquaintances will sometimes buy a game in order to play with their group of friends.
An extreme example of this is Neverwinter Nights 1, which lasted years longer by the power of it's MP and I can account for slightly shy of a dozen direct sales, some of whom I know had a knock on effect and some of their friends also purchased it. Closer to our genre, same thing happened with Civ 4, although to a lesser degree, I think I can only personally account for 4-5 additional sales outside my own, but again, there is the knock on effect, and more importantly still, this is multiplicative across all the various multiplayer hubs of activity. 
In any event, I do hope that MP is considered more seriously for this title; ideally at release. If it is going to come in the form of an expansion, have a reasonable bundle offer right from day one. Makes it easier to on sell. 