In reply to sib_m: It's not a case of either or, if you want to do it then you will choose a route that works for you.
I left a previous job and went back to Uni to do a relevant Degree (Outdoor Education in the Community). This gave me some theoretical knowledge but at least as importantly (if not more so) gave me time to build experience, get a feel for what career paths there were and to pick up some (crucial) NGBs. The placements i did were also vital. Meeting people already working in the outdoors let me learn a great deal from them and working hard with them meant i was offered work when I finished my Degree.
Equally I have friends who started with Traineeships and have gone on to being Freelance or Full Instructors.
Your temperament, academic bent or otherwise (a huge number of Outdoor Instructors are dyslexic and avoid academic work) and financial situation will all have an impact on the route you choose. But any can work.
My own perspective (and its only mine):
NGBs are essential to getting employment.
Experience well above the minimum required for NGBs will help you pass them. I notice a big trend of people coming for Assessment with just the bare minimum requirements. They can often produce set piece solutions to situations but the lack of experience shows through as soon as you as a question that needs them to apply basic principles in anything but a basic fashion.
When hiring people I was always cautious of those straight off of 'fast track' schemes. They often had the NGBs but little or no experience having only the minimum requirements.
Work with lots of different people, question what they do to understand the approaches they use and the tools they apply. Then through this understanding develop your own style.
You are only as good as your last piece of work. Everyone knows everyone in the outdoor world. If you adopt a professional approach to your work, work hard and seek out opportunities for experience and to improve word will get around slowly. If you let one person down or do one poor job everyone will get to hear about it. Your reputation will be at least as important as your CV.
Don't expect to get rich.
Do expect to trash your body so start taking care of it now (cross train, don't ignore minor injuries- seek treatment and advice, use trekking poles etc.).
Don't do it if you want to do climbing. Do it if you want to work with people whilst going walking/climbing/mountaineering/paddling/biking etc to help them get what they want out of the experience.
If you are going to specialise you had better be very good at your job (note: this may not be the same as being a good climber).
The day you think you have nothing more to learn stop and think again.
The day you stop looking forward to going to work get another job.
G'luck.
Al