Also you don't need a flight plan, that is completely optional (it is essentially a SAR service if you crash). With modern avionics you can dynamically recalculate flight paths on the fly in the cockpit.
You really don't need clearances either, unless flying into class B airspace (which you don't really want to do that with a small plane anyways). You can fly across the country without ever telling anyone.
Also having separate planes and cars has real disadvantages. What about when you arrive at the destination? You always have to rent a car. Not only that you have to store the plane which can be quite expensive. There are many advantages to this.
You absolutely need a flight plan. You do not need to file a flight plan, but you still need a plan. You need to know where you're going, how far it is, what the weather is like, what the weather is going to be like over the course of your trip, what your alternates are, what NOTAMs are in effect, etc. This step is, again, not optional if you enjoy living.
The price premium of these roadable airplanes will pay for decades of hangar rent and car rental, no exaggeration, so I don't see either of those as a disadvantage.
How does staying in the same vehicle help? Is it really that big of a deal to move between your car and your airplane?
Let's not forget that airplanes need a preflight inspection before you fly if you're not suicidal, and having just driven the airplane to the airport doesn't eliminate that need. Quite the contrary: being exposed to the stresses of driving will increase the need for a preflight inspection.
The time-consuming non-flying activities for an airplane are preflight inspection, flight planning, clearances, etc. A roadable airplane does nothing at all to help this.
Maybe you save two or three minutes out of your half-hour preflight activities. Whoopdedo. Meanwhile, you make up more than that lost time because your dedicated airplane is a better and faster airplane than the crazy "flying car".
Sometimes you're really just better off having separate objects doing what they do best. I don't see any push for amphibious cars, for example. Everybody seems to have accepted that you're better off buying a car and a boat separately. But somehow there's an obsession with airplane/cars. I blame The Jetsons.
I could be wrong, but I'm going to need someone to give me some sort of actual advantage to one of these things before I might think I am. It doesn't save you time, it doesn't save you money, so what's the point?