That might not necessarily be a bad thing if implemented correctly. There was a Nintendo DS game called Race Driver: Create & Race which its biggest selling point was a course maker much like the one you've described. What was cool about it was that you also got a choice of scenery options and objects which made the courses more enjoyable.
That was the problem I had with GT5's course maker; the tracks never really felt like proper tracks, and there wasn't a whole lot of imagination/variety you could have with them. If PD added the ability to drop in some track side objects and features such as buildings, tunnels, even foliage etc., you'd have the ability to made a great deal of track types like city courses, half-decent real world replicas and some brilliant original creations as well.
It would be fine, but not as good as a fully free track creator like the ModNation one. As you say, there's still a lot of things you can do with a tile set type system, and there are potentially advantages.
The thing is, what they've described as the GPS app wouldn't work very well with a tile set type creator. Either the tile set would have to be enormous, or the tracks would bear very little resemblance to the route captured by the GPS.
I suspect we'll get very little freedom with our scenery placement, and I'll explain why.
GT5 courses had all those hills and bumps, and I figure they were so common at least in part to make it so that you could never see too far ahead. This eliminates a lot of the problem with pop in, which was still an issue on some tracks and even more of an issue in the point-to-point rally stages in the career mode (which were almost certainly using the exact same GT5 course maker tech as well).
It's likely that this is going to be a problem in GT6 as well. To some extent, you can still see the track popping into existence on Sierra as well, although much less than in the GT5 rally stages. I suspect that they're going to have to use scenery and buildings and such to creatively block lines of sight to minimise this effect, since if they're giving us full control over the track then they're not going to be able to abuse elevation to do it as they did previously.
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Frankly, I think ModNation Racers is the current gold standard for what a user friendly course creator in a mass market game like Gran Turismo should be. If you imagine it with some more realistic textures, props and backgrounds instead of the cartoony ones then it would be perfect.
Unfortunately, the area available is quite small and I don't see any way that Polyphony could think that they could make something like that as a 20x20km area.