I spent a year without a driving licence due to medical issues while still having to keep up my normal travel load. It was, mostly, a nightmare as I couldn't use taxis for long journeys (red flag to my employer) so public transport was the order of the day. Trying to get anywhere in the US was a total pain and took twice as long as the car journey, although it did introduce me to some interesting experiences I wouldn't have otherwise had, for sample, taking the train from Boston to UNH was peaceful and picturesque, while the freeway is quite dull.

The question for me now is not "should I get a rental car?" it is "shouldn't I?", i.e. if there's a reason why the car would be impractical (central Paris or London, for example) then I don't, but otherwise it's a no-brainer.

In some cases it can be the difference between spending one day or two days in a place. If my flight is 6pm and I owe the client a whole day on site then I can probably make the flight with a rental car, get on to my next site that night and give the next customer a whole day too. If I have to get a taxi to a rail terminal or wait for a bus then I will most likely have to get the flight the next morning and then spend two half days with the next client. The cost of the rental car becomes eclipsed by the costs of the hotels and meals.