I personally don't believe that Venom is Spidey's "archenemy," or rather his primary archenemy. However, one of the things that I feel makes Venom interesting and different from the rest of the rogue gallery IS his similarities with Spidey such as the whole Spider themed costume and Spider themed powers. Whether it was for eye-candy purposes or not, it grabbed the attention of plenty of readers. Whether Venom's story has any real depth is really up to the reader to decide.
Obviously, characters like GG and Doc Ock have filled the nemesis position, GG especially, but Venom seems to fill what GG and Doc Ock lack, which is mirroring Spidey as what Spidey probably could have done as a villian instead of a hero. I feel that, due to the encounters Spidey has had with someone like GG, he only hates them more than fears them. With someone like Venom, I feel Spidey fears more than hates because Venom most usually comes directly after Spidey except for the occasional exceptions. And this is really what makes Venom different GG and Doc Ock.