Notwithstanding the issue of employment contracts, many MP's find having spouses working for them very helpful to their marriage and well-being. MP's work long hours, in stressful environments and while some marriages work well with both couples spending long periods apart, some do not.
I am open to becoming an MP one day (I don't mind if it doesn't happen, but it's a career move I sometimes consider), and if I ever get married at some point in the future the prospect of going into politics in that direction would weigh heavily against me in making such a decision if I want a healthy marriage. That is assuming of course, that I'd be married to someone who would want to work for me ;-)
We have to be careful in what rules and recommendations we put forward for MP's to follow. Many of them are noble in intention, but we must consider what the social emotional cost would be and whether we can be too rigorous as to turn off aspiring new talent from across the political board.