Printer drivers manage color in RGB mode and the printer has it's own, on-board chip that does the RGB-8+ color conversion. This conversion and the screening are high value intellectual property that Epson and Canon in particular have invested heavily in and will hold close to their chest. Essentially all the ink limiting, linearization, etc are built into the driver and are presented as "Media Types." These days it works fantastically well and we should congratulate Epson, Canon and HP for making it so easy for us. When you're buying a large format printer you're partly buying the shit-ton of work they've put into making this work so well. So make some awesome RGB profiles (pay attention to the Perceptual rendering options) and marvel at the quality.
Honestly I wish we had printer driver for more large and grand format printers. UV curable, Epson's Surecolor, Agfa's Ardeco, etc. It's all too easy for the manufacturers to let the RIP handle all that work. Of course when it comes to using white or silver inks that's where the capabilities of the OS drivers end and where a RIP is needed. Too bad Apple and Microsoft don't have what it takes to take the printer driver API's to the next level.
As for the Canon iPF printing plug-in - Canon iPF users love it for it's simplification, speed and reliability. It's the only 100% reliable way of printing profiling targets without color management and the profiles you'll end up making are compatible with both the plug-in and the driver that other applications will use.
When it comes to things like Nesting and tiling - you don't necessarily need a RIP for that. Canon's "Free Layout" app lets you perform basic nesting when using the driver from any application. And there are a new breed of applications (that call themselves RIPs for simplicity) that have things like nesting, tiling, sharpening, Postscript rasterizing, color management, etc that use use the driver to connect with the printer. ImageNest, ColorBurst Overdrive and Qimage are a few of these "RIPs" and they prove how good the driver technology can be. If you're a nesting geek, nothing beats ImageNest's nesting algorithms (which are also included in Overdrive's "Layout" app).
OS Printer Drivers are the future for aqueous printers. RIPs that handle multicolor+white and silver are the future for Solvent and UV curable machines that sometimes use these inks that the printer drivers aren't capable of.