There are alot of advantages of using hashtags, but as this article shows (Obama Campaign Gets Wise to GOP Twitter Tricks – Alexandra Jaffe – NationalJournal.com) it is getting tougher to use them for social media events because of what could be called hashtag hijacking. Key quote:

And, perhaps more significant, it allowed Obama to avoid the GOP hashtag hijacking that has become par for the course whenever Obama has previously announced a social-media campaign ahead of time. Though Republicans did latch on to the hashtag eventually, the White House’s craftiness prevented the hashtag hijack from becoming too much of a social media centerpiece, as has happened with such efforts before.

I’ve seen other instances where activists have hijacked commercial hashtags to counter a marketing message. I would recommend organizations be wary of using them in the future, unless you want to give activists and other critics the opportunity to weigh in. (And if you do that and you disagree, you better be ready to counter their comments.)