I do see your point, kinda, and I am not attending either... I might if I was closer, but I am not sure if I would even then.
However, I also see why people do want to go there. I am pretty sure a lot of them, maybe the majority, are aware it's a "pay entrance fee to a marketing thing", but it also seems to be a lot of fun.
Yes, it is marketing, but you get to be with thousands of other people who play the game you play. That might not be appealing to everyone, but it is to many. I know of some people who are in guilds together who meet up there... sure, you can just meet in another town without any entrance fee, but to be at an event where you and many other players celebrate the game they love to play is a nice extra.

Also, even though it's marketing, it seems to me as if it's not that typical "boring guys sitting at a table showing some statistical diagrams while the audience slowly nods away" type thing... its a celebration of the company AND those games, and I am sure its fun to be right there at the lore-panels, maybe asking questions yourself, or seeing the dancing/costume events etc. You meet like-minded people, and its ok to let out your "inner geek", to talk about the game, have tinfoil-discussions, all that stuff.

Sure, you can meet with other people you play with, you can watch the lore-panels and stuff on youtube a bit later, but I do see why people go there, and look forward doing it, as, even though it is a big promo-thing for Blizz, it also is a fun event for the players, a chance to hang out with bunches of other players and celebrate WOW/SC/Diablo.
AND.. I did watch videos of the announcements for Wrath and Cataclysm, and call me a geek, but I got goosebumps from that... seeing that moment, how the lights go down, the trailer is on the big screen, people cheering, applauding, laughing etc.
I am sure it was a great moment to be there. Not life-changing great, but I bet it was fun, and kinda exciting, so I do see both sides, the "its just a sales-event side" and the "its FUN to be allowed to be a geek and celebrate the game side"