Err, falling behind in development time thus releasing what is effectively a demo at half the price to try and save face is fecking awful.
It's a cynical ploy to keep the coffers ticking over, while trying to rush something that should have been out a long time ago. Can you imagine if all publishers followed suit? It doesn't surprise me that it's happened though, it surprises my more how long it took and how many people can't see the dangers in such a precedent.
Lots of games fall behind development time. I would much rather have a demo of the game I can enjoy for a year for a reasonable price (I consider £19.99 reasonable), rather than waiting for a release of the full game.
Maybe 5 or 6 years ago I would have had a different opinion, but the wonders of updates through the internet means GT5P can be constantly updated (For free), thus you are getting a preview of the game. If you don't like it, you don't buy the full version. If you like it, you buy the full version.
I can see the downsides of it, but consumers don't have to buy it do they?
Plus buy releasing a Prologue it gives us the consumer to give Polyphony feeback on what they can do to improve the game. (The developers have already took quite a few ideas on board, I'll try and find the live blog)