While I really hate dealing with the influx of, for want of a better word, retards, most of whom use the free time to deliberately wreak havoc and nothing more, I still support the free trials. The more people that are exposed to the game, the bigger the community will become and at the moment, bigger is still better. It keeps the game alive and highlights just how big this realism "niche" really is. It's only a niche because realism fans don't know about it.
Not to mention that it builds interest and funding in the next game. And I guess as Ramm mentioned, it pays for all those free updates that people keep getting. Personally I think we've already gotten more than enough and it absolutely astounds me to see the number of ingrates on these boards complaining that there should be more, more, more, all for free. Considering that TWI quite possibly hold the record for the most additional post-release content from any game company ever (excepting MMO's, where you pay for the content every month anyway), I think we've already had our money's worth many times over.