1. GameMaker is a proper engine, not just a level editor. Even if you start from the position that level editors are subject to limitations, GameMaker doesn’t suffer from them.
2. GameMaker is great for beginners. The strength of GameMaker is that it’s simple enough to learn quickly, while still being powerful enough that you can make some engaging stuff with it. I’ve heard of academic game design programs that use GameMaker as part of their introductory curriculum, and I can understand why it’s their choice. If you’re at an introductory stage, GameMaker is a good option.
3. The bundle includes source code. Although GameMaker is as accessible as a serious engine can be, it still involves programming–and that can be a real hurdle to clear. Having solid, working code to start from is a tremendous benefit to those new to programming, and easily makes the higher tiers of the bundle worthwhile.
I’ve only used GameMaker briefly, but I’ve been impressed with what I’ve seen. I think you’ll be more than satisfied with what you get for $12.