Don't agree at all Milly; and RE isn't about teaching diversity; it's ONE way that it can be tackled in a rural school in Cornwall where being ethnically diverse is coming from Devon.
I didn't train to teach RE to partake in the current insidious Citizenship creep in schools. Religion is a fascinating study, that has and continues to have a great affect on many people's lives.
You also do RE teachers an injustice by suggesting that we don't recognise and teach that ethinic diversity exists within a religion. You only have to look at how wide the Anglican communion is to realise the differences between the Evangelicals in places like Nigeria and the approach of some diocese in the US and how much their individual culture informs their take on their religion. For me whilst an Iranian may be Muslim, I also know Iranian Jews and Christians, so I wouldn't suppose anything.
RE does not give priority to that aspect of someone's identity...how can it if one is teaching non-religious students? It's an area of study that takes in philosophy and ethics; that looks at the nuts and bolts of several religions. For me it would be criminal to send students out without a degree of religious literacy and an understanding of what it means to some people to be religious. It will avoid them being arrested for inciting religious hatred by an off the cuff comment at the very least.
I could also argue that Religion is vital to an understanding of history, art and literature, as it informs much of what is out there.
The brief of an RE teacher is not to try to get students to have or espouse a religious belief. That is down to their parents. The job of the RE teacher is to teach the student learn from and about religion - how religion might inform the laws of a country for instance - lots of the laws of the UK could be lifted straight from the decalogue; what the welfare state is based on (the decalogue and the golden rule); the abortion and euthanasia debates; the value of human life; embryo research; hunting; war and peace; prisoners of conscience; criminal justice are all examples of things I've taught. Apartheid gets covered, as does wealth and poverty; non-violent protest is another topic.
If parents want to send their children to a faith school, then that is a matter for them If you have no objection to Montessori and Steiner schools for example, then why object to a faith one? Your children don't have to go to one; why do you have the right to deny that choice to someone who does have a religious belief? The fact that we are even debating this topic means that RE is alive, and well and on the agenda.
Incidentally, if you want schools to be secular, then you would presumably support a ban on hijabs and headscarves in schools, as well as crosses and other religious symbols?