It removes the unusable rubbish from the store.
The last thing Apple (or I as a customer) want is a vast quantity of demo or crippleware on my computer and cluttering up the store.
Demo and full version of the app could easily be combined into one entry and that would solve the clutter problem.
I don't know about you, but even 50€ is a lot of money for me, and I'd like to try the app first and buy it only if I find it useful. And I suspect that the average Joe will not know how to find the demo version on the internet.
I don't mean the crippleware. I was intrigued by an interesting-looking code editor, but I couldn't test to see if it would really work the way I thought it would, and it was too expensive to buy on a lark. Without sharp eyes, you may not see the developer's site link or know to check for a trial version.
