i feel that a knobby in the rear/slick in the front config. absolutely enhances your pedaling.
there is more kick in your rear wheel (legs) and the road is gripped where it counts through all sorts of terrain.
theres gravel only sometimes yes, but sand is all over the road. so i say its not a bad idea in that regard.

Front tires should be slick and fast for fine steering, which is the point of the front wheel, unless you need to hook up. a slick in the front is fine in everything but mud, because its your rear wheel via your legsthat hauls everything so it needs more grip.

With this mix, Your ride will also not be as smooth unless your tires are pumped at high presh, then there really is no difference in ride quality. just a more reliable tire that does not mess around.
goodluck
it all depends on the situation.

simplest