I have a straight stitch only machine that I use strictly for piecing quilts. It gets quite a workout! Then I have two combo (sewing and embroidery) machines, one that gets used nonstop, and the other occassionaly... it is my backup machine and class machine.
I have a vintage Singer that doesn't get used, although it sews beautifully, just because I wanted it... and then a serger and a stand alone coverstitch machine.
Finally, I have a commercial longarm. All these machines (except the Singer) get used quite regularly, some daily. I could never have just one machine, as sometimes machines break or need servicing, which means that machine may be down for awhile.