The argument here centres on the situation in UK, which is quite different from anywhere else in the world. The airline sector here is very, very large compared to almost any other country, with a huge number of operators all looking to get pilots as cheaply and reliably as possible.
There used to be a well-established pecking order, which saw pilots moving up the 'food chain' as they gained experience, and convention generally required that they had gained considerable amounts of hours and experience outside the cosy world of dispatchers and fully-equipped downroute stations before they graduated to scheduled passenger jet flying. The only exception to this was the BA Cadet scheme, which utilised a training regime similar to the RAF's to bring on baby pilots who were ready to occupy the right seat of an airliner with only a couple of hundred hours.
A few years ago, regulators, FTOs and one or two airlines thought, "Well, if BA can do it, so can we!", and devised a number of other ways to get people into jets at an early (cheaper) stage. This has crystallised into the integrated (or CTC's 'structured modular') system, and has altered the market to the extent that many UK airlines derive almost their entire ab-initio pilot input from one or more of the 'big four' schools. The main reasons are, as alluded to above, cost and risk.
As Redsnail implies, no airline recruits only low-houred people; they need a range of experience, and some (like mine) prefer not to recruit anyone without several years and many thousands of hours flying experience. However, increasingly, airlines are insisting on the training risk having been minimised before they will take on a new pilot - hence the insistence on TRs with some line hours for recruits who have not gone through the airline's own quality-assurance scheme - i.e. their contracted training providor. Even BA, who no longer operate any kind of cadet scheme, will take you on with only a few hundred hours - so long as you have proved to someone else that you can do the job. And that's before they put you through their own selection system.
Sidthesexist no, passing the exams and flight tests does not mean you are suitable airline material. More and more, airlines are taking the attitude that their selection procedures are an insufficiently fine filter to weed out the high-risk candidates, and are insisting that the risk is moved elsewhere. For a low-houred pilot, that usually means they want to see a training history, right the way up to a TR, that demonstrates the ability to learn quickly while under pressure, and sufficient handling and thinking ability to go on to command a jet airliner in as little as two years from being hired. That's a big responsibility - and the major risk is taken by the employer if you screw up.
Yes, of course there are still airlines that take pilots with instructing experience, who went the modular route and took their time about it. We get several success stories from career-changers and the like who are pretty much forced to go this route, and good luck to them. But this is a percentage game, and wannabes who are looking for the most reliable route to airline employment deserve to hear the truth, however unpalateable it is to those of us who would prefer it were otherwise. And that truth now is that the vast majority of low-hour pilots who get airline employment with 300 hours or less in the UK currently do so via integrated or structured modular courses. There are variations on this theme, of course, and there will always (I hope) be alternative routes. I'm not saying that any given route is less valid than any other, but if you're a young wannabe looking for the best chance of getting a right-hand seat in a 737 within 2 years of starting training, this is the way to do it.
Remember, I am commenting on what I observe, not on how I'd like things to be or whether the currently most successful route is the best. You are, of course, free to disagree with me, but I think you'll find the numbers are on my side.
Scroggs
P.S. Incidentally, the MPL is A Good Thing for the airlines and those who wish to work for them. It is not appropriate for those areas of flying far removed from the rather regimented airline world, and will not be used for those areas. I see no need for rabid disagreements about it, it is simply a case of designing the tools to better suit their purpose.