Answers to those questions can vary. I'm just getting into fly-fishing. If your interests are only in fly fishing for trout in lakes, a 5 or 6 wt. An additional line, a sink-tip to get wet flies down if the trout aren't rising is very appropriate. The reason here is the size and weight of the flies used to fish saltwater.
Bigger flies require a heavier fly line to cast the fly.
Yes, of course it is more work to cast a bigger rod, but we are talking real economy here. Those of us who have fished a number of years all fished with rods that were considerably heavier and more physically demanding than today's rods. It's a matter of frame of reference. New rods for specific uses - like salmon fishing, or small delicate rods for small streams. In many cases the pack rod is even better than it's 2-piece counterpart.