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.