1 A) One’s wife has medical conditions and she cannot fulfill the husband’s needs/desires.
B) There is a widow who has no one to take care of her and she needs to get married.
C) There are some women who have no men to marry them in their area, and if married men don’t marry them they’ll never get a chance to get married and have a family life.
D) Political reasons (as was the case with the Prophet): you marry a woman from a tribe to neutralize the tribe and stop them from fighting you.
2) Yes it can be a legitimate reason (if it doesn’t cause major problems with the first family). A man probably cannot love two wives equally equally, but he can still love them both.