Personally, for this game, it doesn't bother me. For some games I would consider this virtually mandatory, for others, not so.
We seem to have developed this model of comparing a game against a checklist of 'essential' features to determine their worth. We don't criticise FPS for not having 50+ levels with secret exits. Different games and genres focus on different things.
I can absolutely understand individuals being disappointed in the absence of this feature because, perhaps, the only opportunity they have to play multi-player with their friends is online. Where i stop short is where people claim that a game MUST be globally scored lower because of the absence of one of these so-called essential features. It may affect the appeal of the game to you but it may not detract from the game for many other people.
Another example is those individuals who insist that Metroid should have any kind of multi-player. You are of course entitled to your opinion........but in this instance you are wrong and should crawl back into your hole! :P