I'm an US expat living in my husbands country, and my spouse is a non resident alien. I need to file unfiled tax years and I want to file married separately, but my spouse will not provide his ITIN. What should I do?

If you file separately and your spouse does not have an ITIN, you can write "NRA" in the field for the SSN. If you want to claim an exemption for a non-resident alien spouse you need an ITIN.

I disagree about the need for the ITIN on a MFS return.

I have successfully filed returns using the NRA option and HAVE claimed the spouse's exemption WITHOUT the ITIN.

Now, if they file JOINTLY, then FOR SURE an ITIN is required.

The IRS clearly states, that if you claim an exemption for a spouse, you need to have an ITIN (if you don't ave an SSN). There are always cases where you get away without, but you shouldn't be surprised if the exemption gets rejected without an ITIN.
http://www.irs.gov/businesses/small/international/article/0, id=96696,00.html