Did I need to get the MBA? No, not really - I'd probably have the job I have now at the same rate of pay. What I didn't know about ROI, payback and IRR I may have had to find out by trial and error rather than by sitting in a lecture room. But to be frank my online MS Excel training courses (done mostly on the train to and from work) have proven to be more valuable to me.
The MBA has come in handy, though. It's like being given the key to some arcane lingo - the language of business. Now you can get this key from a mentor, or from a book or a website. Or you can get the MBA, the key and the piece of paper. The advantage is that you have that piece of paper, and that helps to "prove" that you have opened the door. You still have to demonstrate your competence on the job, though. If you already have a good job and simply want to progress then get a mentor instead and do some private study. However if you want "in" then some prospective employers will expect to see that piece of paper first.
So would I do it again? Yes, if only because I enjoyed it, got to network a bit, proved to myself I could still achieve educationally and - most importantly - got a stack of great text books to keep.