Once Farnsworth and Wernstrom created their diamond-like minerals, it's possible that from that point on diamonds might not be as valuable, although this isn't referenced in the show in any way. Prior to that, diamonds clearly still have value, or at least huge diamonds have value, as evidenced in the episode "Where the Buggalo Roam".
With respect to Bender's behavior in throwing out a sizable diamond (in "Three Hundred Big Boys"), I always interpreted that as a joke illustrating just how much Bender's sensibilities can be warped by his criminal and debaucherous desires. When he sees something he really wants (i.e. Le Grand Cigar), his first thought is to try and get it for practically nothing (offering $300 even though it is priced at $10,000) and his second thought is to try and get it by theft. The fact that he has a giant diamond that he could sell/trade for the cigar doesn't even register, even as he uses the diamond to steal the cigar. He'd rather burgle to get what he wants, because it's a simple solution and, really, it's just more fun. ;)