I remember trialling the XMLSpy home edition (no longer available). It had the very basics, but on each day one of the 'power features' was available (and the next day a different power feature would be available). That way you got a chance to try out all the functionality, but clearly you didn't have all the benefits of the full product.

Reminds me of the movie channel in the hotel room...you get to see a few seconds of a film at a time, but to see the whole picture you've got to pay!

In short I think vendors should be more creative.