There will be installations that will not allow the use of wall-mounted appliances, due to the size of the space.  An example of this is the floor of a convention center.  The use of wall-mounted appliances alone is not practical due to the large spaces needed to be covered, and the presence of large, tall display panels that will be brought in during exhibitions, such as the NFPA Exposition.

In these cases, the use of ceiling mounted appliances makes perfect sense, and is allowed by NFPA 72.  Tables in NFPA 72 outline the allowable spacing for these.