At the risk of being brusque, this whole thread centers on what seems like a rather silly question--or rather, a question that derives from some rather strange premise(s). It might be more profitable to be debating about how lineage could possibly not matter.
As far as I can imagine things, there are only four basic reasons someone might study a martial art at all:
1) As a means to self-defense and fighting skills
2) As a means to the broader kinds of self-development wrapped up in the budo ideal
3) As a means of maintaining contact with the living past--being a part of living history
4) Because it's fun to play with sword or spear or whatever
If your interests are 1 or 4, lineage probably doesn't mean much--if anything at all. Then again, why bother with a teacher at all for 4--you could just as easily make things up yourself or copy moves from movies--and why bother to study anything (except possibly jujutsu) in the koryu syllabus if your primary interest is in 1 (other than as the first step toward 2)? How often are sword, spear, or naginata skills likely to come in handy?
If your motivation is largely 3, on the other hand, there's little room to even ask questions about the importance of lineage. What could be more important, in that context?
And if you're after goal number 2, the importance of lineage and legitimacy is almost as obvious. In this context, lineage and legitimacy assure that you're involved with the real process, rather than a knock off--in traditional metaphors, that you're following an established path that does lead to the top of the mountain, rather than one that's new and untried. That's not to say, of course, that new paths can't be found (or cut), or that the well-documented koryu have some monopoly on The Way. But if you're following a guide who's blazing a new trail, the only way you can be surethat he's headed in the right direction is if you already know the terrain yourself--in which case, why would you need the guide?
The bottom line, I would argue, is that if you're not concerned about issues like lineage and legitimacy, then your interests aren't really in koryu at all; they're in something else. Not, to borrow a line from Jerry Seinfeld, that there's anything wrong with that, but the koryu phenomenon and the appeal thereof is pretty much defined by questions of lineage and legitimacy.
Karl Friday
Dept. of History
University of Georgia
Athens, GA 30602