I think the #1 trait of successful essays is not just stating a goal, but stating a
plausible goal along with a plan on how you are going to get there. For example, you might really want to do investment banking, but if you have no finance experience and have spent the last 4 years saving the Uruguayan tree frog, IB is going to be a tough sell to an adcom. They want a marketable product.
It would be a lot easier to write about how your time studying frogs has taught you that you are really passionate about environmental consulting (i assume that is a real industry) and you need an MBA from X school so that you can learn the business side of things, they have a great environmental consulting department, etc. Once you're in, it doesnt matter what you said you wanted to do on your essays.
I'm not saying lie...I'm just saying, at least in my case, I think the key factor in my successful applications was stating a goal that seemed achievable and logical, even if it might not be
exactly what I plan on doing.