Another thing to consider is wear. Cotton generally weathers at the same rate - after a hundred washings, it will mellow evenly. Adding a poly that will not fade through time is going to give you a different effect. With a bright color, it will be more noticeable.
Another thing some poly blends do is pill up. Anyone who has ever bought cheap sheets has learned to regret this. Cotton is great, and if you have a huge stash or access to what you need, is generally preferred.
That being said, I'm all for blending fabrics for wall hangings and things that will not get heavy use. Sometimes you need a bit of glitz that cotton can't provide. Sometimes the perfect color/pattern/price just isn't available in cotton. Some of the stretching problems can be minimized by using a fusible interfacing to stabilize the stretchy fabric.
Like most things in quilting, it's all up to you.