I agree with Slim. some beer styles are generic enough that a variety of yeasts would work.
but some styles require the right yeast be used to fit the style. if you don't use a hefeweizen yeast, its NOT a hefeweizen. Wit yeast adds a certain 'something' to the beer. It will be beer if you use another yeast, but it won't have all the subtle flavors that help define witbier and separate it from other wheat beer recipes.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10