As they come and go, they won't confine thier browsing to the food put out for them. Another problem with feeding deer is thier acute sense of smell. They will detect and become accustomed to the human scents associated with the food source. This will make them less fearful of humans and they will come much closer to houses than they would have otherwise. Feeding may cause them to associate inhabited places with food, but they may already have figured that out.
Deer learn quickly.
Since deer have a home range of about two square miles and ranges overlap, you wouldn't want to feed too far away or too close. Incidently, hay alone isn't adequate deer food in the winter. In the fall they stop producing an enzyme needed to digest cellulose, which means the nutritive value of the hay won't be realized. They will eat it and slowly starve. Feeding with anything else could get very expensive and also attract other wildlife and be consumed in even larger quantities.