If I am not mistaken the problem with marrying a low skill wife is that your heir will end up with much lower skills as well. From what I understand, your children get a mix of stats based off of you and your spouse so lets say you have 100 1-hd skill and your wife has 10 1-hd skill, your child might end up with 30 1-hd skill vs if your wife had 200 1-hd skill, you child might have like 120-1hd skill. Again I could be wrong as I haven't died in any of my campaigns yet but if this is true, a low-skilled spouse just means your going to be starting from scratch with your heir.