All great points, Frilly. Plus new plants might be discovered in the spring, such as spring-blooming bulbs that would not show up unless you inadvertently dig them up. You really don't want to do that, as an established clump of daffodils or tulips is a treasure.
You can get some awfully good deals on perennials plants in the fall, though. When I lived in Utah, and now plant gardens there for my kids, we plant a lot of perennials, and especially shrubs and trees in the fall to give them the winter to establish roots. This does work great, the plants do well, and you're paying 30% to 40% off when you're buying them. It's just quite important to mulch them really well to regulate the soil temperature and moisture through the winter.
