The reason for the shortening is for the texture - shortening contains millions of tiny air bubbles that expand from the heat in baking (about 20% of the volume of shortening is air). It also has a higher and broader melting point than butter. If you creamed the butter and sugar and then refrigerated it for about 30-minutes to firm up a little and then cut it in as the shortening it might have a minimal impact, but oil will definately result in a denser muffin.
Believe it or not - if you use the new Trans-Fat Free Crisco, it's actually healthier than butter - lower in saturated fats and no cholesterol. And, believe it or not - butter contains some trans-fats!
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain