I mean, take a look at her. No, not the giggly, easily-muddled Billie Burke depiction. She's considered the most powerful person in Oz, has a book that gives her real-time updates on everything that's happening, and yet she never lifts a finger to help anyone unless they ask first.
In The Wonderful Wizard of Oz and The Marvelous Land of Oz, she seems to be aware of the events of the adventurers, but waits to act until they come to her. And what's with all those spies she mentions in The Marvelous Land of Oz? Who's to say they don't still work for her?
And does it seem that after Ozma comes to power, she seems to consult Glinda often? And what about Glinda seemingly acting for the good of Oz when she's been unbidden? Take the end of The Emerald City of Oz. She cloaks Oz in the Barrier of Invisibility (which doesn't really keep people from entering Oz, taking The Scarecrow of Oz into consideration) before Ozma asks. Why? It's obvious. She wants to protect Oz from outsiders...
..But the big question is, who will protect Oz from Glinda? That's the same reason she sends Queen Ann and the Oogaboo army outside of Oz in Tik-Tok of Oz! And of course she'd go rescue Ozma and Dorothy in Glinda of Oz. It gave her a heads up on some new magic, and it got some other magic-workers out of the way!
And her training the Wizard? Of course she would! He's her apprentice. She needs a follower.
Think about it, she knows everyone's strength, everyone's weakness. Something sinister in brewing in the south, and no one in Oz sees it coming...
After a few comments, I want to note that I wrote this firmly tongue-in-cheek, but I think my point is valid: if Glinda ever went bad, virtually no one could stop her.