It is not necessary. I don't have it installed, and probably never will, as it is just another thing to keep on top of when updating. Especially when the information for updating is a bit cryptic (who needs it, who doesn't

)
whether you personally need it or not will depend entirely on your needs, some sites require the java platform, and I think, a few of these may be games. You will find out if you need it when you come across such a site...
So far, I have only come across one site that requires java, and that is secunia's online scan, and I just didn't bother with it...
-Scott-