See discussion in https://github.com/apache/arrow/pull/1852#discussion_r179919627
There are various Append() overloads in Builder and subclasses, some of which append one value, some of which append multiple values at once.
The API might be clearer and less error-prone if multiple-append variants were named differently, for example AppendValues(). Especially with the pointer-taking variants, it's probably easy to call the wrong overload by mistake.
The existing methods would have to go through a deprecation cycle.