As someone who had to return a lavaliere, here are my thoughts:
The guy that I was dating lavaliered me, and I did take it very seriously. He also bought me a set of letters to wear. The problem was he started grad school about 9 hours away, and we just drifted apart. Another problem was that he was 4 yrs older than me, and he just couldn't identify w/ how much enthusiasm I had toward sorority life since he had grown out of the Greek scene by then, and that caused problems when, for example, I wanted to celebrate w/ my sisters for winning Greek week for an hour or so before coming over to see him. But I digress...
I broke up w/ him b/c I was miserable, and the first thing he did was ask for his lavaliere back. He told me I could keep the lettered shirt. Which a sorority sister of mine borrowed (since she was also a little sister of his fraternity) and never gave back. I think that was a conspiracy on her part.
Anyways, I think that a lot of girls identify a lavaliere/pin as a sign of a serious commitment, so when they say they want to be lavaliered by graduation, they're saying they hope they're in that type of a relationship by then. But I'm sure there are also a lot of girls who just want to see how many boys they can get to give them their letters.
I also think they're just letters, and sometimes people take the symbol of letters a little too seriously. For example, I think there was a thread around here somewhere that a girl was asking if it would be inappropriate for her to wear a Lambda Chi lavaliere that belonged to her mother...and IIRC (thanks muagd - I think it was you who told me what that means!), the man that lavaliered her mother wasn't even the man she ended up marrying! There are also plenty of threads discussing pin collectors owning a GLO's pin if they're not a member - I think it would be even less of a concern for a lavaliere b/c it's not even the org's badge!
As to the level of commitment that a lavaliere represents, I personally consider lavaliering a pre-engagement type of deal...whereas others consider it equal to engagement. Right after I got lavaliered, I was at a mixer w/ another fraternity, and one of the brothers jokingly commented that he couldn't talk to me anymore since I was engaged. I was like...."What?" I did notice that boys talked to me less at parties when they realized I had on a fraternity lavaliere.
Perhaps that's why so many lavalieres are returned - b/c it's seen as something that's easier to break than an engagement. Also, perhaps the people in the relationships realize that they're not ready for such a large commitment - lavaliering is sort of "practice" for being engaged. I personally would rather have to return something the boy spent $30 on than something he spent $3000+ on, but that's just me.