There at the bottom of my colleague's email was this passage from the Greek philosopher:

We are what we repeatedly do.

Excellence, then, is not an action, but a habit.

So the virtue of writing every day, if we can be serious about it, if we can make it part of a cluster of habits and virtues: listening to others, thinking about what exactly they are saying and what exactly supports their perspective, testing our own ideas and experiences against those of others and without prejudice, and so forth.

And so the vice of writing every day, trying to win arguments no matter the cost, ranting at others, stacking the evidence, calling names, forgetting what you don't want to acknowledge, ridiculing differences, testing everything against your own superior beliefs, and so forth.

If that's right, both ways, then blogging is a medium and a practice that gets quite a bit of its virtue or lack of virtue from what it associates with. That means that we don't really get anywhere at all urging people to blog -- we'd need to urge them to take up a whole package of practices and attitudes that wrap around blogging and make it special. It's not blogging (or tweets or whatever the newest cool tool might be) alone. It's a whole package deal of virtue.

But blogging has going for it the repetition that Aristotle called for. That's one of the necessary ingredients of character.