The C++ driver (or whatever DBClient is) expects a truthy value: https://github.com/mongodb/mongo/blob/3779bf419438d2625277cf3c1c7216c497f683a8/src/mongo/client/dbclient_rs.cpp#L256

...but the MR execution code only cares that it's a key: https://github.com/mongodb/mongo/blob/3779bf419438d2625277cf3c1c7216c497f683a8/src/mongo/db/commands/mr_common.cpp#L72

@jessedavis asked about this earlier and our take-away was that the [`out` option](http://docs.mongodb.org/manual/reference/command/mapReduce/#mapreduce-out-cmd) must be either a string or an object. If you specify a string, it's obviously the output collection. For an object, it might either be some action (e.g. merge, replace) or inline output. For inline output, the value of the `inline` key is irrelevant. Suppose you specified `false`. In that case, you still haven't specified an output collection, so where else can MongoDB dump the MR results? I believe that's why the server only cares about the key. Much like when we run commands that take no arguments (e.g. isMaster) -- the value is irrelevant.