I don't really see it as pointless because the symbolic gesture of promising myself to somebody for life is what means something to me and doesnt need a government stamp nor fit in with their definitions so would just feel odd to include that part. It would be like a nonreligious person using vows that included God, I don't really believe in the government defined marriage but I believe in my lifetime with DP.
Then you wont be husband and wife.. which is what you said you wanted to be called.

I too believe in marriage equality but not god so mine was a civil celebrant ceremony. I believe that a commitment ceremony which is what you have described to me is done in place of when a marriage is not legal (ie. same sex couples which I do believe should be allowed to be married but thats a whole other thread)

I just also think if you want to be called husband and wife and are going to all the trouble of the celebration and vow exchanging then it would be rather silly to not do the legal part to get the husband and wife title.