In some ways I like to take things apart and try to figure out how they work, so picking through the JS is just some sort of habit I guess. 
And who's to say that all this dissecting of undocumented features won't change under the hood with DS 1.3 or later (thus undocumented) -- That being said, sometimes you just gotta do what you gotta do in the now. (Just putting a little disclaimer on my research in the event SAP swoops in and says this or that isn't a best practice -- Well, duh! )
Regarding the DOM, yes you have access to the entire thing -- Both a blessing and a curse, in a way. With JS, you must self-police in many ways, seeing as there are no strong means of protecting the integrity of objects or preventing overriding of methods, or even wiping out DOM elements, or other bad practices.
Like other JS UI frameworks, I think playing with the DOM directly instead of the UI framework that may be involved in tracking element IDs, etc is a precarious situation, but probably has its place for certain scenarios, such as triggering a pop-up or whatever that you clean up with after, etc. I've been trying to play by the rules where possible with using the same UI lib as the application, so I've gotten to learn a lot about SAPUI5 along the way. (I prefer Sencha's ExtJS, personally)
For my usages (including a fully blown chart wrapper for Highcharts I'll post about next week), I stay within the constrains of the generated DIV, so that I can utilize the width/height/margin positioning that the IDE facilitates for me, but yeah definitely nothing stopping one from playing around with the DOM for fun or to learn how things work.