Having a free trial does not exempt a downloadable game from taking advantage of PR; not even in XBLA, where every game gets downloaded by a large number of people. Why? Two reasons. First, that "large number of people" could be a lot larger. 2x (or more times) larger, in fact. Just because a lot of people download every game that comes their way doesn't mean you can afford to ignore the people who don't. Plenty of consumers only think to download the titles they are familiar with -- that's why licensed IP is so popular with many publishers.
Second, conversion rates are influenced by anticipation. This is easy enough to understand. Imagine being faced with two games, both of relatively equal quality. One has been hyped in the press for months. One is unheard of. Your friends are all talking about the first game. You yourself have been looking forward to it. But the other game is just as good. Which are you going to buy?
The trial... it's not just the first five minutes of your game
It's surprising how many developers don't think about their game's trial experience until the very last minute of the development process. A downloadable game's trial is everything! If someone doesn't enjoy your trial, then they probably won't buy your game. It doesn't matter if you licensed the three greatest IP of all time and fused them into the holy trinity of game design itself. If the trial stinks, most people won't bother to lift the curtain on the full experience.
* Don't confuse or frustrate the player
* Don't make the player wait for the fun
* Don't make the trial too short
* Don't make the trial too long
* Intensify the player's curiosity
* Work that upsell message
* Show off your best features