Cho's E

I've played a little bit and now I've seen him being played in the lcs competition. Something that I have pondered since day one that I have played is why is Cho's E a toggle? I can't imagine a single person who doesn't put turn it on and leave it on. I don't know the benefits of having it off or even if there are any. Furthermore I think I would like to see in the future his E removed and replace it but have his Vorpal spikes as his aa. I could be wrong and there could be players who know the pros and cons of having them on and off in which case please enlighten me.