Is it worth renting a car to explore some cities one way and then dropping car and taking train back to airport?
It depends. What trip did you have in mind?
Car rentals are cheaper when they're not picked up at train stations or airports--city locations. But you usually can drop them off at airports with no upcharge.
The main thing is not to drop them off in a different country or you'll pay dearly.
Ditto, it depends. You rarely need a car for a city itinerary. Using it for rural travel to villages in between cities is fine. I have never found a one-way fee within a country or with a city pickup to be even noticeable.
Check carefully for the drop-off charges which can be hefty or not allowed for certain places. We had 2 separate rentals in April and it was far less expensive to drop off at the same location (Rome-FCO and Florence airport). A certain large company overcharged me for an upgrade which was not requested, but my persistence got it reimbursed.