In my opinion, The best way to keep you from neglecting something that looks like a "nice to have" task in your project is by making it part of the process. If you can make your diary based on your commits/bug system's tickets or even features of the project, you might be able to keep your diary up-to-date while blogging only on the important things of the project. Good luck ;)

Upsides are all great... If you start, though, stick with it. It becomes a marketing deliverable just as important as anything else, since it is public-facing. If you choose to abandon the project, you just wind down the blog. Just remain consistent about content delivery - if you promise to blog once a month, make sure it gets done. Make the blog easy to find and connected to the rest of your project's public website. Transparency for users has nothing but upside, provided you respect their attention and commitment to your project.

If find this a very good idea. In fact, I want to have a public diary of all my development and learning activities. I don't see how it could backfire, quite the opposite: if you abandon the project at least you blogged about problems you encountered and what you have learned and so on. Instead of nothing (project gone) you have delivered postings