Daniel Finkelstein has an excellent post up on the realities of parallel Sharia law: The conversion of non-Muslims near death without the knowledge of their families has caused fierce rows. One reason, according to my correspondents, is that conversion changes the destination of any inheritance with Islamic courts deciding and inherited assets flowing only to Muslim relatives or the community.
Divorce battles raise similar questions. Conversion by the father in the run-up to a divorce gives him crucial advantages - he gets custody, turns the children into Muslims and prevents his wife using the civil courts.
Running a dual court system produces extraordinary practical difficulties and the opportunity for human rights abuses. Just ask the campaigners in Malaysia.