Yeah, ya know?
Well, that's how it works.
It's nice to understand that a person does know, or is aware of, or has felt a particular way about a particular situation.
But just shooting out the same old one-liners again and again starts to defeat the purpose. I had a conversation with a guy today, ya know? Well, no you didn't. But now you do, because I did, and I told you about it. This man decided (for whichever reason) that 'I knew' (ya know?) everything he was talking about, and stopped about halfway through every other sentence to let me know that, er, well, I knew.

1) If I already knew, why'd I have to listen to him for so long?

2) I didn't already know. Heck, I'm still not even sure what the point was. I dismissed him with a 'Well, that's how it works' having no idea at all what was working, or how it was intended to work.
One of my bosses at work just randomly throws in a 'ya know' whenever she stops in the middle of a thought. It's her trade-in for an 'and uh...'.
Now, in addition to this, you get to factor in the fact that (many) people pick up on the broken pre-'ya know' fragments as something they're supposed to know.
"Well, Harley's vibrate so bad, ya know? It's just crazy."
"And so, like, I couldn't get jack for a trade in on a japanese-bike, ya know? And.."
Now, there's three things that happen when a 'ya know' is dropped:
1) Disregard it - You don't know, but you don't care enough to bother telling.
2) You do know - Or you at least get the point (or think you will).
3) You don't know - But, because he/she assumed you would, you'll just take it as a fact and say you know (assuming he/she actually produced a complete thought before asking, ya know?)
Now, in the case of #3, I think this is how a lot of bad stereotypes get started. People latch onto info they think they should know (without being sure of it's source) and regurgitate and vomit said information whenever possible, so as to hopefully look somewhat enlightened on a topic they might not otherwise know anything about. I think the biggest 'ya know' factor is the need for social acceptance, really.
Now, anyone reading the 'Question for Harley owners' thread, don't go and flame me. This isn't about you.

Ya know?
-Nick.