I would argue that "not a plugin" and "not controlled by a single corporation" are more than "irrelevant". Here's a few reasons:

- Want to port your game to a new platform (game console? embedded device?) - just get webkit/mozilla/<your favorite open source browser> to compile. Although this is no small task, in many cases it can be easier and way faster than rewriting the code for an entire game. not to mention this mandates one port per console, rather than one port per game per console. N^2->N time savings :D

- Not being controlled by a single corporation is incredibly important. Take iphone games: want to update your game to use some newly released feature of the sdk? Get ready for 1+ weeks of waiting for approval. Notice a low-level bug in the sdk? ha-ha, have fun talking to Apple's technical support. These are the obvious reasons, there are a million-and-one examples as to how limited access can slow down - and even halts - the game development process.

That's all I have time for now, I'll try and think of some more later

Edit: too/to