Currently you can hint to use an index for range+equlity matches but the index will not be considered otherwise. I would like to see it be considered like any other potential index.

Example of desired behavior:

db.coll.insert({a:twoDaysAgoDate, b:2})
... // millions of docs where a is less than 2 days ago
db.coll.ensureIndex({a:1,b:1})
db.coll.find({a:{$gt:oneDayAgoDate}, b:2}) //uses index, doesn't now.