Are marriages doing anything to halt violence in Westeros? Freys married into the Lannisters, then went to war with them (a war the Lannisters started), Florents married into Baratheons, then fought Stannis and his Florent wife, Joffery gets betrothed to Sansa, then starts a war with the Starks, and the Brackens and Blackwoods are commingled enough to start a new house, if not for all the war with each other. I know there are more. Is there a point to people getting married for peace anymore?

Well, historically, it is a successful strategy – the Stark Kings bound much of the North together with marriages, the Lannisters and the Gardeners prevented war with the Andals through marriages, the Gardeners brought the Arbor and Oldtown into the Reach by marriage when war failed, the Rhoynar assimilated into Dorne through mass marriage, etc. 

I suppose the major distinction is to what degree these marriages are embarked on by both sides (and I mean both the Houses and the married couple) as a means to peace.