I always use a straight stitching on my big quilts and FMQ on smaller items only because I find it stressful doing FMQ and I take a long time to pluck up the courage to do it - therefore time wasted.
I feel when doing straight stitching the machine is set at a length and my stitches are nice and even and I have a good "straight eye" when using my walking foot. Sometimes my stitches when FMQ - especially around curves -are not as good as I would like and I am not happy to put them on a project that I have spent such a lot of time on particularly if it is a gift for someone else.