There are some good reasons, and some really stupid, desperate reasons why corporations and other types of large organizations are fearful of entering the blog world on their own behalf. Here are some good reasons you may have for not blogging:

1. You can't sustain it... If you already have a more or less full-time communications, PR, or marketing job, unless you have boundless energy and are a workaholic, you likely won't be able to sustain a corporate blog on your own. If you think you can assemble a team of co-workers to help out, unless they are very much into blogging, it rarely happens. If they are very much into blogging, it still may not help. We had a client that assembled a team, and the team was into blogging, but they kept posting to their own blogs.
2. You really don't have anything to say... If this is true, you've got bigger problems than deciding whether or not to blog. If you can seriously look out across the competitive landscape, the market opportunity, your product differentiators, your unique selling proposition and the rest and decide you don't have anything important, unique or compelling to say... I'm begging you, don't blog.
3. You're afraid the unvarnished, transparent, authentic truth could ruin the company's reputation or your career...This is a legitimate fear. When done right, blogging introduces an element of candor, authenticity, immediacy and conversation that stands in stark contrast to the safety of prepared, approved, sanctioned, one-way communications materials. Think of it this way: traditional marketing communications is promotional; blogging is emotional. Unless you have a senior-level champion of blogging, it's unlikely an organization with a classic, long-standing approval cycle mentality will quickly and easily adapt to the more free-wheeling style and candid substance implied by blogs.
4. You're not convinced there is a 'big enough' audience for your daily ramblings... If you're in a corporate world in which big traditional "total potential reach" numbers are important, you or your management team will be very unhappy blogging. Corporate blogging isn't about broadcasting a one-size-fits-all message to the most possible people at the lowest possible CP/M in hopes of actually reaching the ones who might pay attention. Blogging implies a significant corporate cultural revolution because the real-life, real-time analytics you get will deliver a set of acute data points proving whether or not anyone is interested in your message. That's a very difficult transition not every corporation is either willing or able to withstand.
5. Your executive management team doesn't get it... If this describes your situation, stay away from the whole blog opportunity. Successful blogging involves writing and posting some things that traditional senior management executives find most uncomfortable. In fact, much of the traditional corporate communications apparatus evolved over the past 50 years has been aimed at preventing exactly the kind of talk blogs favor. In the midst of building a solid, loyal, avid, hard-won blog readership, someone on your board, an investor, your boss or your boss' boss will make a call or a comment to someone and that will be that.
6. Your executive management team is OK with it, as long as everything passes through the same approval cycles... This won't work for at least a couple of reasons. One is that the immediacy and timeliness of blogging is lost. Two, something happens to vivid, compelling copy on its way through the traditional approval gauntlet. The message goes in edgy and comes out the other end sanitized and sanded smooth of texture. For a corporate blog to work, someone has got to trust someone to be smart enough to routinely write about important things in an appropriate way, in their own way, while everything traditional gets out of the way.
7. You have no experience as a reader of blogs... To make an analogy, it's unlikely you can write a good book if you've never actually read one. Or, it is best not to learn how to tap dance in full public view. If you haven't yet become a routine reader of blogs and are not using them as a source of news, information and opinion online, you should do a little of that first or your initial blogging experience is apt to be clumsy and embarrassing and that's never good in the corporate world.