The hebrew word for wife 'ishshah' is the majority of the time translated - wife but has also been translated 2x as harlot, harem 5x, harlot 3x, and simply woman 1x (Strongs Exhaustive Concordance). I agree with the answer above from J.C. Sa*lomon regarding ANE practice but I would add that they didn't have a word for "surrogate", at least I don't see that word in any biblical translation of the OT. In antiquity a woman who bore your children was your wife in the broad sense of having your child, but not in the actual sense of being the woman you "covenanted" with for life. I might add these slaves could be sent away (like in the case with Hagar) or sold and the children of that slave kept because they were the owners property especially when they were the 1/2 blood of the master. In Ishmael's case in Gen 21:9 Ishmael was "mocking", textually in reference to a feast they were going to have because Isaac was weened, so Sarah wanted him to leave as well. God also confirmed this with Abraham, so thats what he did.
These woman bore children to these men by the initiation of the wife (not to assign blame just to make a point) Sarah said, "I will obtain children through her". I conclude by this account that Sarah believed the child would be hers since Abraham was her husband and Hagar was her slave. This proves that Hagar wasn't on par or equal to Sarah no matter what the term used to describe Hagar was. The full text makes this clear.
For Jacob it is basically the same thing except Jacob had two equal "covenantal"wives (For lack of a better term) - Rachel and Leah who both had slave woman who bore them children.
If you look at Gen 30:10-13, in Rachels eyes they were her children.
She said concerning herself,
(these statements had nothing to do with the slave woman who bore the children):
Obviously these maids / slaves were not wives in the sense of having a "covenant" of union with the husband, they were simply surrogates for the wives.