Interesting idea, I'll give you that. I imagine you could achieve a basic RIP of sorts, but to create a user interface like Command Workstation, or incorporate features that directly feed into the digital press would be a huge challenge. Take for example changing toner dither/dot pattern, image shifting or enhancements, Calibration - Come to think of it, a solid 90% of all EFI's job properties would be difficult to replicate.
Of course, if you don't care about the output, a basic RIP would save you a considerable chunk of change. Just simply use the stand alone print driver.