Currently, unnest operator is not required to specify alias neither for table name nor column name. This has some implications on what name the unnest operator output column should use. One can use a common name like "unnest" as the output name. It means, customers need to be educated on what to expect from unnest operator. This might confuse some customers and also prone to introduce errors in the query.
The design decision for DRILL is that unnest always produces either a scalar column or a map (depending upon the input schema for it), but it is always a single column.
Given this scenario, it is better to enforce the requirement that unnest operator requires a table alias and a column alias(single column). This can help to disambiguate the column and further can easily be referenced in the query.