I'm curious so am going to ask, not to stir! How important is it to practitioners that the history is 'real' or is what you train in more important?
Obviously I don't do ninjutsu so it may seem a heretical queston if so I apologise but I have a huge interest in all arts. The subject of 'lineage' seems far more important in some countries that others, in the UK it's rarely mentioned in any style as what you are learning is more important than who your instructor's instructor was. On MT I see Americans consider it very important.
Where do I start....
There are several angles. Chris would no doubt be much more eloquent and verbose than I am going to be, but let's see what I can come up with.
First of all, traditional Japanese arts encompass a set of techniques and guiding principles. It would take quite some time before you got to learn the 'good stuff' so to speak, with the advanced teachings reserved for a handful of disciples who would receive menkyo kaiden (mastership license) as proof of that. Only someone with that license is considered to have received a full transmission and complete understanding.
As long as your lineage goes towards an unbroken line of menkyo kaiden holders, you have a guarantee that you are learning the real thing. Plenty of people drop out halfway, or get kicked out, and set up shop someplace else and claim to teach their original art without having the license. But because of not having that license, it is a guarantee that they will not teach the complete art, or even the art in a correct way.
Second, if someone is claiming to teach 'ninjutsu', he should be able to prove the lineage. Because without it, it is a certainty that he has not received a transmission of the original art, but either teaches an incomplete art, or something altogether made up. Apart form the things I mentioned in the previous paragraphs, this also means that the person making the claims is likely a fraud. And personally I wouldn't want to learn from a person like that. Keep in mind here that we are not talking about the fighting prowess of the teacher or the students, not about the effectiveness of the teachings. This point is solely about completeness of the system as originally passed down, and the integrity of the teacher.
I don't mind people who created their own ninjutsu system. For personal reasons I think it's ridiculous to use a Japanese word with a specific historical context to describe a modern system that bears little resemblance (let alone fit the cultural context) of the original. But people are free to choose whatever name they want. Many of them are open about this, and more power to them. But claiming to teach the original while it is a lie... that is a low thing to do.
But let me reiterate the main point: in traditional JMA, lineage is important because it guarantees the completeness and correctness of the system. In modern arts, lineage is far less important because it doesn't matter if you change things, as long as they are effective. I've also practice modern jujutsu, and lineage was barely mentioned at all, because that was unimportant. The name jujutsu was simply used to describe a system that had Japanese roots, and got things tacked on from different sides. all that mattered were the results.