I see what you mean, although it would be all the more reason to have this ability at our disposal. With a large enough map, having options to how we get around it quickly is always a plus. Nomura says the game will encourage exploration and have hidden secrets (treasure, rare items, etc.) So it seems to have been part of the plan since day one. No doubt there will be a limitation on its usage earlier on into the game. But you know how it goes, as we level up and progress so will the abilities.
Well we aren't actually saying that the map will be as detailed as a GTA map, but that the means of traversal and design could be similar to a GTA game. No actually game currently has a 1:1 realistic modern city (not even GTA), so no one is expecting that level of detail. Also it wouldn't make much sense not be able to drive throughout the city when there are cars and roads on the very map.
The cars even have detailed interiors and can get be damaged meaning that they're interactive. If that weren't the case that would be a serious waste of resources and assets.
Based on what we've seen from the announcement trailer and gameplay trailer, I honestly do think we'll have a large scale of exploration in the town.
If last gen consoles can run games like GTA V and Just Cause 2 with their scale, the PS4 and Xbox One should easily be able to handle what we've seen so far.
We have seen cars in the city of Lucis, and based on past trailers of Noctis actually fighting in the city, down what looks to be a major street and also back alley ways, it wouldn't be surprising to me. Remembering back to when Noctis was on the bridge when explosions were going off, Nomura said if you continued to go down a certain path on the bridge a chest would be there.
We have yet to see any game take advantage of the power of next-gen consoles, but with a look at upcoming games, where they have cities and interiors to many of the buildings you would want to interact with, it would be no surprise if you were able to explore these cities to a great detail.
Take a look at Accordo, based on the map seen in the announcement trailer, you can assume you'll be able to go everywhere in that city for the most part that we've seen visible thus far. Does that mean every house is enterable? No, likely not, as not all houses in previous Final Fantasy games were enterable, you cannot expect that. I do expect that it will be possible for a lot of areas to be explorable however.
In terms of the world map, comparing it to 2D Final Fantasy games in 3D doesn't match up directly to what we've seen in past trailers. Without any new information on how this will work, we cannot say for sure the scale of the world map, or the transition between the world map and the cities themselves.
I'm expecting that with the recent information of the game being "quite far into development" we can expect to hear more information in the coming months, and likely some form of release information at E3 this year. I'm hoping to see information around the launch of the PS4 in Japan, but that's just wishful thinking.