Spider Jockeys are just as useful as any other hostile mob, just more like a mini-boss sort of thing instead of a regular mob.
Moving on, I don't think there is any reason to ride spiders, we already have pigs, and horses are on their way, as well as minecarts. While they can't climb walls, spiders would be near equivalent to pigs. Furthermore, it seems illogical to ride a spider, they aren't exactly the friendliest creatures. Maybe it's just because I hate spiders both in real life and in game, but it's a no for me, sorry.
I like it, but I think that they should be tamed with Rotten Flesh or Silverfish meat (as stated above), and then a saddle can be put on after. They should be brown with blue eyes once tamed also. And, if you don't feed it enough or it is exposed to darkness for a certain period of time it will drop the saddle and become hostile again.