No, it's not too rigid. Some DM's are too rigid. Some players are too rigid. But alignment isn't.
The biggest single problem I've had with it is that its presence can reduce intrige and mystery. There are only so many times you can dangle an evil aligned character who is innocent because the real killer is neutral, or hide the real villain behind an expensive magic item that conceals alignment.
I think most of the problems can be solved by making alignment detecting or motivation revealing divinations less certain, or by making mortals have so weak and uncertain of an aura that 'detect evil/good' spells don't work with them.
As far as the alignments being vague, I've never really had that problem and it seems to exist mostly on the internet. I can imagine the sort of player that would get in an argument with the DM that his chaotic evil character was lawful good, but I've never encountered one. I'm sure its happened to someone, but never me. Yes, alignment is vaguely described in official rulebooks, and yes the problem is made worse by the fact different designers and writers disagreed over what alignment meant or else didn't think very deeply about it when creating characters or societies. Yes, authors have made matters that much worse by promoting in various fashions contridicting claims about alignment. But all that confusion means is that any given DM has the freedom to define it how he likes, and also generally accept any players internally coherent definition of the alignment as well.
People generally agree what "friendly" or "violent" means.
Do they? I would think that if they could generally agree to what 'friendly' meant, then they could generally agree to what 'good' meant. In my experience, there is relatively little debate over what 'good' and 'evil' mean except when it comes down to specific moral prohibitions. Most of the debate seems to be over the more specialized fantasy terms 'chaos' and 'lawful' which have been portrayed in the broadest variaty and with the most contridictions.
Likewise, what alignment was King Arthur? Sounds pretty easy, right? A lot of people would quickly contradict you.
I'm not sure that that proves anything about alignment. It may just prove that people have different information about King Arthur and different images of him in their head. There is no real reason to suppose that the King Arthur in my head is the same as the King Arthur in your head. In order to even have a hope of any sort of agreement, we'd have to ask alot more specific questions like, "What was the alignment of King Arthur in the third section of 'Once and Future King'?" And my opinion wouldn't count, because I got bored with the book about then, skipped a hundred pages or so and read the end. Before I could give you a valid opinion about a specific King Arthur, I'd have to be an expert in the text that establishes his character. Alot of the problem with alignment threads where we argue the alignment of various fictional characters is that we often just have vague impressions based on a few half remembered things, or that we are using different sources or what not. Asking the alignment of King Arthur is like asking the alignment of Batman. The first question you have to answer is, "Which one?"