Definitely a want vs. a need if you have unlimited data plan and a smart phone. For our recent drive down to FL we purchased a newer Garmin (less than $100) just because we are used to having the large screen sitting on our dashboard. I enjoy seeing if I can beat/knock minutes off the arrival time. It is easier to use if there is just one person, but you would be just fine with a smart phone/maps app.
I agree with the PP that the GPS traffic information was pretty useless for us. Not much better on smart phones though, from our experience.
Also check data coverage in the area you are traveling. Never a problem along major thouroughfares, but in small/rural towns/areas you would be better served with a Garmin/satellite reception vs. smart phone/data reception.