You won't want the car to work downtown. Parking and traffic suck and it's expensive.

You won't want the car if you live in or near downtown for the same reasons.

You may want the car if you live a little bit further. About halfway to the ends of the T lines, it might be worth having the car for getting away from the city. Use the T for work and use the car once every week or two to go hiking, skiing, etc.

Once you are off the T, you'll probably need a car for daily life, but you may want to take the commuter rail to get to work. This means driving to the stop, switching to the commuter rail, then switching to the T and doing the whole thing in reverse in the evening. You could also drive the whole way, but like I said in the beginning, traffic sucks and parking is expensive.

The choice to bring the car depends on your lifestyle, budget, and personal preference. If you're big into outdoor life, then living further away and having a car would serve you well. If you're looking for city life and are happier walking through museums and parks then living closer to the core and skipping the car would be better.