There are a few answers to this, alot will depend on what you want to achieve and who you want to work with/which market you wish to service.
For example a top body builder working in a body building gym can make a good living working with Bodybuilder and Fat Loss Clients whilst not having to learn the latest ACL recovery techniques. Whilst the "Industry" might frown at their lack of qualifications, they are just hitting their market and using their knowledge base to make an income.
If you are looking to get into rehab and sports specific training or working with special populations then a qualification is more essential so you prescribe the correct programming and dont harm your client!
PT wise, Personality also comes into play, if you are an outgoing person and can easily converse with people, getting clients is alot easier. If you are the strong silent type, even if you know training and programming inside out, getting clients and imparting your knowledge onto them will be more difficult.
Qualification wise, if you have years of practical lifting experiences perhaps in Power Lifting, Oly Lifting or just have tight technique and have been taught well as a Body builder, some of the short or block courses will help with the theory and basic principles of exercise and touch on periodisation, small business management, marketing etc.
Richard Dryden (Former NZ Olympic Lifting Coach) was like this, he was a great observer of human movement and was coaching the NZ team without a paper qualification behind his name, he went and got these later on in his career.
Another thing to consider is whether you want to be part or full time. If you want PT'ing to be your sole source of income it is bloody hard work and takes a massive amount of effort in marketing, retaining and councilling clients and following up payments , diets etc. It can really suck the life out of you if you dont qualify your clients carefully!
Also, People tend to forget, you can be the best exercise coach in the business and still be broke unless you put equal effort into the business side of things, an old quote: Your pile of Books on Small Running a small business should be as high or higher than your pile on exercise.
I agree with Jordan in that A Sports Science Degree (if time isnt a barrier) is the gold standard is you want to coach athletes at a high level and/or get into sports research etc however there are always exceptions as their are some excellent course providers out there who can give you all the tools to do this as a profession as they focus mainly on Personal Training and small business management.
Some of the most successful (Financially successful) PT's aren't the most knowledgable PT's but they know marketing, time management and running a business and can spot opportunity at a glance. Whether purists agree with this approach or not these trainers are business people first and trainers second.
I think what Jordan was referring about PT certificates being devalued these days is that their are too many PT course providers in the market now and many have questionable content with some requiring no hands on practical assessments which I disagree with. These leads to a poor product and brings the industry standard down.
Personally I went through NETFIT whihc I found excellent, this was run by Tina Ball (NZ Oly Lifter) and with PT coaching from Tamara Middlenton-Eschave who I found to have a very in depth knowledge of the industry, At the time I believe she was the only PT to be on a direct Government Referral basis with ACC.
NETFIT also bring in Tony Boutagy from Aus for their Strength and Conditioning Module.
Motivation plays a big key as well, Those who arent motivated but think doing a course will be the magic bullet will invariably fail in the industry, personally I was really motivated to do the course and pass well as it was an industry change for me ( I HAD TO SUCCEED) and I studied very hard, enjoyed every minute of it and got everything I could out of it, The tools I gained from NETFIT plus heaps of my own study and research allowed me to become the busiest PT they had ever had in the 2 Gyms I worked at.
One thing I almost forgot to mention, Just as you mentioned about viewing yourself as an apprentice, thats a good analogy as no matter how hard you study or how much you know going into your qualificiation, there is always more to learn so reading a massive amount of the latest research, going to conventions with like minded people, PAYING for some sessions with other trainers from differnent backgrounds who you would like to learn from, these aspects are all very important to your growth and development. That said not every latest trend or study works out to be correct and many are discounted almost as soon as they are published so my philoposphy was to take on board and assimilate what you agree with and disgard what you dont agree with.
Just my 2 Cents.