This is a much more complicated relationship than you make it appear. Sarai 'gave' Hagar to Abram (his name at the time, names are important) specifically so that she would have the child that Sarai could not have (at the time). I am not sure what constitutes a marriage, officially at that time, but clearly Hagar was a wife, just not the 'first' wife. This is a very human story. Sarai perceived that she was lowered in Hagar's estimation (because she had no son to give her husband), so clearly Hagar's thoughts and opinions mattered to Sarai. It is not Abram who initiates casting Hagar and her son out of the camp. And an angel speaks to Hagar and giver her God's promise to make a great nation out of Ishmael. True there is a rough prediction for Ishmael and his descendants, but that goes for some of Jacob's (Israel's) descendants as well.
And have they not been borne out?

Interestingly, I have heard Christians denigrate Ishmael as a 'servant's son' but I have never heard Jews do so.