Honestly, I have seen it done - but unless the facility has an excellent orientation program, I don't think it's a good idea. Reason being, you really need to be able to think on your feet and learn to prioritize quickly. You need to get used to being pulled in 10 different directions at one time. You need to be competent and comfortable with your skills (assessments, rhythm recognition, blood draws, IV placement, NG/Salem sump placement, foley placement, etc.) so that you can do them quickly and correctly.
I have worked at hospitals where the orientation for new grads wasn't great, so it was the rare few that did very well on their own within their first year. Where they had great orientation programs (classes/clinical with excellent preceptors), the new grads seemed a lot more comfortable/confident and were able to take off running after orientation (which is needed in busy, understaffed EDs).
I also think it's nice that new grads experience floor nursing first so they get to see "the other side." They will understand why they can't send their patient up to the unit right away, they will understand why the floor nurse can't take report right when they call. They will learn to prioritize and will get the chance to perfect their skills without being under the extreme pressure of having to do it all on 5 different patients within a matter of minutes.
That's just my two cents on it, and I'm sure others will disagree with me - but I was an ED nurse for over 10 years and those were my honest observations.