The debate on wether or not it is ok to use cultivars is always a hot one. Native plant gardeners typically fall somewhere on the spectrum of 'Only straight species' to 'Any unique cultivated plant will do'. We all need to do our own reading and decide what, if any, place cultivars have in our gardens.
Luckily Mt. Cuba center in Deleware has started to evaluate cultivars and you can find their research published here: http://mtcubacenter.org/research/trial-garden/
What is a cultivar and why should I care? Cultivars are simply asexually cultivated plants. Some are even derived from naturally occurring populations. They are cultivated via cuttings or other non-pollinated means. This preserves the exact genes that give a desired trait, like resistance to powdery mildew. Thus all cultivars are genetically identical. Straight species plants or open pollinated plants are usually grown from seeds with genetic material from two parents and thus each plant is unique.
At Watermark Woods we do carry several cultivars(all pesticide free , of course). Cultivars do have merit. For instance, In a small yard a dwarf cultivar may be the perfect size. Or, who wants a plant covered in powdery mildew when there is a resistant cultivar that is still extremely attractive to the pollinators? Some folks may be just fine with the powdery mildew as it is purely a cosmetic problem for the plant.
Our stance is that if you choose to use cultivars in your garden, please plant a variety of the species. That way we are still preserving diversity.