I find philosophy blogging so useful, fun, and worthwhile that I'm surprised there aren't more academics (esp. grad students) doing it. Admittedly, I'm an enthusiast. But I take myself to be enthused by reasons that other philosophers could share. For instance:
(1) It helps me do more philosophy -- exploring and writing up ideas that I would otherwise never get around to. (One could use a private journal for this purpose, but I'm sure the public aspect helps boost and maintain my motivation. YMMV.)
(2) It's a great way to get critical feedback on your ideas. (As previously argued, blogging is a medium well-suited to philosophical discussion.) This point will become all the stronger as more and more top young philosophers join in.
(3) It disseminates your insights and ideas to a broader audience. This is worth pondering. If you love philosophy, no doubt you also care about sharing it -- correcting common misconceptions, helping others to come to a clearer understanding of difficult problems, etc. So I cherish the fact that this blog has received over half a million visits, many from undergrads and philosophically interested layfolk, in addition to the fellow grad students and academic philosophers with whom I engage most.
Conversely, I'm very appreciative of other bloggers, from whom I learn a great deal. I only wish there were more! -- sharing ideas and summarizing helpful things they've learnt, so I could learn them too.
So, why aren't there more academic bloggers? I expect the main factor is sheer inertia: people don't tend to start doing new things without some reason. (It takes time and effort, after all.) But I hope that drawing attention to the above factors will help instill some readers with the requisite inspiration.
Others, however, may be positively resistant to blogging. Public speaking is of course a very common fear, and I wouldn't be surprised if some (esp. less confident students) have a more generalized fear of (intellectual) 'public exposure', being judged negatively, etc. I don't have much to say about this, except to wish such people luck in overcoming their unfortunate fear, and so dispelling this 'attitude-dependent' reason not to blog.
Are there attitude-independent reasons to fear blogging? I've heard two proposed:
(A) Fear of getting 'scooped', or "giving away ideas without getting much credit". Brian Weatherson had an interesting discussion of this a few years ago. As he points out, there's more to professional reputation than publications alone. (And for many people, at least, blogging will tend to help them produce more publications in any case.)
I would add that it's not entirely clear to me why blogging should be thought any "riskier" than other ways of doing philosophy in public -- e.g. conference presentations -- that most academics wouldn't give a second thought. But if you're really worried about someone poaching your dissertation topic (say), then I guess you could always just restrict your blogging to side-interests. So this worry is - at most - limited in scope.
(B) Fear of hurting your chances on the job market. I take it the idea here is that your half-baked thoughts aren't as impressive as your finished work, so putting the former on display may create a worse overall impression for Hiring Committees. I guess that's possible, but we need to consider (i) how likely it is that HCs would be irrational in this way, and (ii) how this balances against the likelihood that blogging will help you professionally.
All else equal, I take it, people tend to be more well-disposed towards familiar names and faces, and participating in blog discussions is at least one way to get your name out there. (Compare the professional/networking benefits of attending conferences.) Plus there's always the chance that your thoughts might positively impress.
Notably, even if your blogging alienates more people than it impresses, base rate considerations suggest that it's still overwhelmingly likely to be beneficial on net. Suppose, for example, that a Hiring Committee checking your blog has a massive 10% chance of being discouraged from hiring you, compared to a 2% chance of being encouraged to hire you when they otherwise wouldn't. The crucial observation is that the base rate is overwhelmingly weighted against hiring you to begin with: let's be optimistic and say that as many as 10% of your job applications will be successful by default. How does blogging change the default chances? Well, 1% (10% of the originally-inclined 10%) will be discouraged from hiring you, and 1.8% (i.e. 2% of the originally disinclined 90%) are shifted in your favour, for a net gain of .8%. So your overall success rate increases from 10% to 10.8%, even given the most unfriendly assumptions. It's easy to see that this happy effect is magnified if we are more pessimistic about our default chances, or less pessimistic about the ratio of impressed- to unimpressed blog readers. Plausibly, then, blogging is more professionally prudent than not. (Though anyone motivated by this reason alone would probably not do a very good job of it!)
Overall, then, I think there are some pretty compelling considerations that count in favour of academic blogging, and ungrounded fears counting against. Such weak concerns may be enough to maintain inertia, if someone isn't much fussed with the prospect of blogging in the first place. But I think it would be a real pity to let vague professional paranoia prevent you from doing something you consider truly worthwhile.