You can just donate the socks to a charity. New socks and underwear are always needed at places like shelters and organizations that give clothes to people who can't afford them. I do think it's really wastful as long as you can find a way to use the item, even if that is giving it away.
All the time! I make sure I get something I can actually use though and its not something excessive I don't need at that time. Like you, it's usually small like a mouse pad or a pair of socks or something. Especially with how easy it is to lose socks! But its not like you can't use those things and getting the $10 rebate is far more important in my opinion. I
I generally avoid this...how much of a deal is it if I'm buying things I don't want or need to get it? Now if it's something I'll use eventually or something I could see myself gifting at a fair price I might consider it but overall I just think that it really defeats the purpose, spending to save.