They are necessary when handling Postscript data and CMYK data. Otherwise, they are not really necessary. There are some workflow advantages too but with products like Lightroom and its print module, the lines are blurred.
You're right, they're not necessary, but there are some other important perks that those you mentioned. RIP's provide access to a greater number of quasi-customized profiles than are provided by OEM's such as Epson (which only includes Epson profiles). Also, and most importantly for me, when printing work in grayscale a RIP can/will drastically improve the output quality (the Epson grayscale option for the 3800 is nice, but cannot compare to the Imageprint RIP).
The bottom line is, as always, cost vs. benefit. If you need customizable and repeatable layout packages, access to a large number of very good profiles and/or the best b/w digital printing that money can buy, then a RIP might be a good choice, despite the high costs. If not, then save your money and by the misses a nice gift.