I will agree -two machines are better than one lol!
I have a straight stitch only machine I do my piecing on, and often am using my combination embroidery machine to embroider at the same time. However for the few times I need something like a zig zag or decorative stitch, I can use the embroidery machine.
I do have a couple of other swing needle machines I can use if I'm in the middle of an embroidery project. I have used three machines when I'm making some embroidered crazy quilt blocks and needed to also do some zigzagging of embellishments using invisible thread.
Some of the combo machines come with neat editing and design capabilities. I don't know if the embroidery only machines have that, or if you would do your designs on a computer and then simply upload them to the embroidery machine. I just bought a stand alone design program, so probably won't use the one built into my new embroidery machine much.