I like what Jaxcat had to say. Under Construction (placemakers journal) did an article this month on hiring apprentices, it points out the fairly obvious on what to look for when hiring, although directed at builders it is still relevant for plumbers.
I have worked for a few firms and 10 years ago when I started my apprenticeship I had the most generous boss I ever had by far. I was in a van doing maintenance/ small reno in my 2nd year with a phone, travelling allowance, bonuses, better wage than a rego'd plumber starting rate nowadays and we had in house night school (not paid), beer fridge always stocked, every 3 or 4 weeks we would have reps and trainers come out for demonstation evenings on new and current products and alot of tradesman we could rely on albeit over the phone as Jaxcat said. Yes there was admittedly alot of mistakes and it was hard to learn out there on my own, to a young guy it was great but looking back now I wish I'd had more support on the job, it took me alot longer to build up confidence because of my predicament. So now I view my training as very substandard.
That said, times have changed, from my viewpoint it seems taking on apprentices is riskier than ever, yes they make you money but with companies out there doing large works for zero profit, it doesn't surprise me apprentices are doing very low risk work unsupervised whilst the plumber in charge is hurrying to meet his labour quote.
At the end of the day its swings and roundabouts I suppose, you have to give if you wanna get and if the guy your working for is just a plain asshole then the grass really is greener