A question on low-req items

As I understand, low-req items are more valuable.

Is this simply because they are rarer - i.e. they are 'collector' items that people like to have?

Or is there a practical reason why I'd want to buy a low req weapon? I see people talking about req 8 weapons, but if you have a character that uses that weapon, wouldn't you want the weapon stat to be higher than 8 anyway?

Both reasons are true. Low req weapons are prestige items, just like elite armor. The functionality is pretty much the same (with a few exceptions), but it's harder to get.

Performance-wise, there are some circumstances where low req is useful. Sometimes you can afford to put 6/7/8 points into an attribute for a shield. Even though the AL will be lower, meeting the req will grant more armor than a max shield without meeting the req. Regarding your example with weapons, you are correct, but as above, it's a prestige thing.