Critical PUT semantics call for setting to null any properties not specified in the payload. PATCH semantics apply the specified properties but leave unspecified properties unchanged.
Specifying that clients ALWAYS use PATCH for updates is much safer for round-tripping data. For example, open properties, aggregated values, additive schema changes, and computed projections could all add additional properties to the payload that the client should be able to safely ignore. If the client uses PATCH to update then they do not lose information, but if the client ignores properties and uses PUT to update they will likely lose data in round-tripping.
OData clients should always use PATCH when doing updates.