Ask HN: How important is "unobtrusive javascript"?

I have read in many books that all javascript should be "unobtrusive". Its said to be best practices to avoid the problems of traditional JavaScript programming (such as browser inconsistencies and lack of scalability).

But here I see world's biggest search engine which handles zillion, jillion, and gazillion of web traffic are not following this "best practices" and it works just fine

See image: http://i.imgur.com/q9N4n.png

Further, unobtrusive javascripts requires bit of extra effort(adding event listeners etc). for user's perspective it works the same way.

I guess, the number of people do not follow these "Best practices" are more than who do the extra effort and follow these Best practices. In both cases, apps work just fine.

so question is:

How important are these "Best practices"? And why should we follow them knowing smart people at google don't? they must be doing this for a reason, right?