Phoon's experience reminds me of the old saying 'There is not such thing as a Free Lunch'
Pardon me for stating the obvious, but what did you expect? 
Businesses are made of people, and people need to make money to pay their bills and keep on funding development of the stuff they do. Everyone does.
So, free offers of commercial software only happen for two reasons: the one you stated (the hope that users will upgrade to the next version and pay for it) and/or increase exposure of the software. This should be pretty obvious, as it is only common sense, and, besides, it is also made pretty clear on the GOTD site if you read their Terms of Service. Not like it's hidden information. 
As I discovered, as a software developer, the problem with GOTD type of promotions (opposed to Bits Du Jour, for instance, where the software is still sold, albeit at a heavy discount), is that it comes with a 'hidden' heavy price for the software developer as well:
First, the whole thing is a nest of software pirates circling GOTD like flies waiting for something new to crack. Second, some users fail to read the instructions (GOTD Terms of Service clearly states that their promotions offer NO re-installations and NO updates, and that you must read the included readme file, as applications promoted there, being from different software companies, are installed and registered differently) and, instead of owning up to their own mistake, blame and insult the developer for 'leading them on'. And this even when you offer them a heavy discount - when you have no obligation to do so - to become a 'legit' user of software they actually like enough to have been using every day for the past two months.
It's agravating, to say the least. 
