Generally speaking, allowing for $in queries to be able to handle a richer level of query power such as ranges would be immensely helpful. Otherwise in some cases where $in should be applicable, you are forced to use $or and hence forced to pay a performance cost.

Something like the below would be what would be nice where a new query operator for that field, would also be accepted.

find( { a : { $in : [ { $gt : 5,  $lt : 10}, { $gt : 20, $lt : 30} ] } } )

The above should return all values of a that are between 5-10 or between 20-30.