I'm not sure of the effort required to create all these materials for teaching people all about heroes and items but sometimes it just seems completely half-assed.
well, it's obviously incredibly time consuming recording, processing, and uploading all those videos. they do a sufficient job of demonstrating the tool-tip in a visual way, I believe that's enough for most people to get a quick glimpse of how it works. I don't think it's meant to be anything more than that. you can get more verbose information elsewhere.
such as Dota Cinema.com
. it has longer videos for each hero, that explain the skill beyond the tool-tip and is generally a lot more helpful. but those videos are too long to include in the client, where people might just want to see a quick demo of a particular skill.
watched some of the defense. turned on purge (didnt know tobi was on). he said a crystal maiden should get vanguard early against anti-mage. I then stopped watching purge.