The general 12 week scan can just be used as a dating scan, if the hospital offers nuchal fold then they will check the measurements while carrying the test if you tick yes for that option, but you can say no to it as well. At my hospital you would then have a scan still, but they wouldn't do the measurements.
Whatever your view, it's probably a good idea to think about what you would prefer to know or not before you go for the scan, especially if you're a bit older and/or your local hospital offers testing to you/everyone. First time round aged 37 I wasn't sure but agreed to the test, and my results came back low risk and I thought no more of it, this time they came back high risk - but still 1:120, which sounds ok, not so scary, until you're there! I had CVS this morning so am waiting nervously for results, and wondering if the pregnancy will proceed ok, worrying about miscarriage etc.
My test was a combined nuchal fold scan and blood test - the nuchal fold was fine, the blood test increased my age risk which was already designated "high risk".
I don't think only women who definitely won't continue with an affected pregnancy should have the test, but I do think the decisions involved are very difficult and I kind of wonder if I'd been happier if I'd declined or never been offered the choice.