Curious what the advantage is (in general) of sparklines over, say, a line of circles with filled in = win and empty = loss? you could use a red fill color for streaks of 4 or more. It seems to me a solution like that would have less clutter and better communicate the data...

A single line of circles allows for less information density. If you force it to have the same information density then you lose a lot of legibility. Secondly, it's possibly harder to see where outliers are. I find this especially true when the vast majority of the dataset is 0, and you're looking for the outlying 1s.

See this example I mocked up, where each line has the same number of data points: http://i.imgur.com/xRoY41c.png

Thanks, that example illustrates the information density point very well!

Thanks for taking the time to do that.

What does the partial horizontal line indicate in Tufte's example?

Looking at the Bosox 2009 schedule, it looks like the horizontal line indicates home games.

Indeed. 2nd paragraph on the linked page:

"This example is from Tufte's site, and was made in collaboration with Adam Schwartz. The presence of a horizontal line represents a home game."

Would have been better had you used SVG.