Hiring a car in Provence is either utterly unnecessary or absolutely vital: it all depends on what you're intending to do.
If yours is to be a city or resort trip - heading to Nice, Cannes, Monaco, St-Tropez or Marseille with no intention of moving out - then a car will be more trouble than it's worth. You'll spend money for the hiring, the petrol and the parking and your reward will be furious times in traffic jams heading for places where you would have arrived far faster on foot. Or, in the case of big cities Nice and Marseille, by easy and cheap public transport.
If, on the other hand, you are considering a longer stay, with a view to getting out of town into the glorious hinterland, then a car becomes essential.
Of course, you could use public transport for these trips, too - and, certainly, it's worth considering the train for coastal jaunts along the Côte-d'Azur. The rail service is cheap and effective - and the views are wonderful.
For travelling inland, though, buses and trains confine you to where they go and at the times they choose. With so much beauty and grandeur to explore, this is too limiting - so, yes, a hire car is the answer.