I've had friends say, and I believe, too, that you can't really date half a couple. That is, you'll have a relationship with both, regardless of it being intimate.
But, the way I see it there are two kinds of relationships. One, you have two people that date others on the side. In that case, I guess you could have a very minimal relationship with the metamour. Two, you date with the intention of those people being equal partners in the group...and if you really want to be fully involved with your partner's life, then, yes, I think it's absolutely necessary to have a good relationship with everyone involved.