I'm really curious to hear opinions on this because it's a topic I struggle with myself at times.
On one hand, it's a very nice way to see many of the world's stars prior to the season starting and is almost like a mini multi-cultural all-star game on most nights. On the other hand, not all of the MLB stars play in this tournament and there were a few players on the US team that I could not even name whom they played for.
There are some more detailed reasons on why the World Baseball Classic is a good thing. Probably most importantly is for many MLB stars - it gives them the chance to compete at a high-level of competition prior to the season starting. You look at Derek Jeter, for example, who after the 2006 WBC had one of his best years offensively. Could this have been coincidence - of course it could have been - but I think for pitchers and hitters alike, the extra time can never hurt. Another very good thing about the WBC is that it gives many international players exposure on an international level. You look at much of the Cuba team this year or the stellar performance of Dice-K in 2006. For a baseball geek like myself, it's exciting to see this young talent and know what could be coming to MLB within the next couple of years.
Now the bad things. Biggest problem I have with the WBC is that the timing of it couldn't be worse for the MLB players. This doesn't just affect the US team like I am so sick of hearing on PTI, Around The Horn, and SportsCenter - it affects almost all of the teams! Many MLB teams do not want their star players going out and over-working their unconditioned bodies. There are a few teams (Tigers, Red Sox, Twins, etc) that will not hold back their players from competing for their country, but many do have worries about their players getting hurt or being overworked - and rightfully so. This leads to another downfall of the WBC is that it is not a true showcase of the entire world's talent. Stars such as Johan Santana, Joe Nathan, Alex Rodriguez, Albert Pujols, Grady Sizemore, etc all were either scratched due to concern or not playing in the tournament.
I think there's a lot that could be done to make this tournament better, but for now it's a great source of good competitive (usually) baseball and for a guy like myself this time of year...it definitely keeps me interested!