Hi Carrie!
I know of many different reasons for using fake flowers, including cost, and they aren't necessarily considered any less formal -- but they are less traditional! If anything, I'd be more surprised to see fake flowers in a home garden wedding, where one would expect that real live blooming flowers would be available and would clash with the artificiality. But some of the reasons people choose fake flowers is their permanence -- you can keep them as a keepsake; their availability -- they never limited by whether they are in season; their versatility -- you can get roses the exact shade of your blue dress, or pure black daisies, or other combinations that nature doesn't provide; and their fashionability -- for example, silk flowers can be acquired with real Swarovski crystal "dew drops" sparkling on their petals!
But tradition (and I!) prefer real flowers for many good reasons, some of which are the opposite of the reasons I gave above. Their very IM-permanence makes them more precious, and reminds us of how important it can be to enjoy the blessings of the moment unstained by the sorrows of the past or the worries of the future. This is an important message to take forward into your marriage. They remind us by their very limitations of colour and availability that everything is NOT under our control and that having to compromise should never blight our enjoyment of good things -- another strong marriage lesson. They are the epitome of lack-of-artifice -- natural honesty, being themselves: and that is how a wife must be to her husband, however carefully she presents her image to the world. And they don't gather dust five years from now as you try to figure out whether to keep them or let them go to their well deserved fate in the trashcan.
Just how traditional do you really want to be? Tradition is inherently not modern; and many of the style features modern brides aspire too are actually as UN-traditional as they are expensive. The floral decorations at my wedding twenty years ago were simple florist's vases (of which the church had collected literally dozens over the years) overflowing with fresh-cut garden flowers offered up by family friends: peonies,flax-flowers, lilies and bluebells. They were lush and beautiful, and cost exactly $0. A couple weeks ago I was at a family wedding where each long banquet table held several bowls of white roses: the bride had bought the flowers in bulk and she and the bridesmaids had simply arranged them in bowls with a gold bow around each bowl. More than $0, but much less than the cost of commercial arrangements (and more traditional: remember that hiring commercial skills is a compromise made necessary by our busy modern lives. Jane Austen heroines ALWAYS arranged their own flowers LOL!)
And, of course, there are other centrepiece ideas that don't rely on flowers, and hence avoid the cost of hothouse flowers without resorting to artificiality. Remember that many brides are trying to take advantage of other brides' experience, in order to make their own wedding unique and individual. Really, to be unique and individual, you have to have the courage to try things that everyone else is NOT doing, and to have the good taste to ensure that your novel ideas are fashion-setting rather than outre. It's always a fine line that relies on a nice sense of style.
Tradition is the living faith of the dead. Traditionalism is the dead faith of the living.
Weddings 
