I completed a 2 year RA before I went to grad school, and I think it was one of the best decisions I made. Not only does it benefit the PI, for the reasons that Ollie stated, but I do believe that it benefits the RA as well. After all, even if you graduate and start a position immediately, that gives you only ~5 months on the job before you will have to write and submit your grad school applications. That's not a lot of time to become intimately familiar with what you're doing, and likely not enough time to get a glowing letter of recommendation from your boss.
And remember, it's not only about getting in to grad school... having that extra year will also give you more experience doing independent work, which can also be beneficial for when you actually matriculate as a graduate student.
It's hard to think about "taking 2 years off," when all of your friends are probably jumping right into med school or law school (or whatever). But, in the grand scheme of things, it's really not a long time. And you may find that some of your friends, down the road, will actually envy the time you took to really plan your career. Personally, I know too many regretful lawyers and physicians!