I can imagine circumstances where you might want to sleeve a piece of MC; perhaps there's a particular section that might be subject to damage, or that you want to 'keep out of the way' and lack support points. Finally, theres the issue of penetrations, especially where you want to allow for future replacement of the cable.
Indeed, you would have to use a sleve if you had multiple cables to address.
I'm not worried at all about the sleeve becoming energized. Here's why:
First, we are not required 'structural metal that is unlikely to become energized.'
Second, there is the jacket of the MC, which is bonded. If the EMT is going to become energized, don't you think that the electricity is most likely to originate in that MC?
If you're a purist, you could assemble a fitting to make a direct MC -> EMT transition. If you're worried about abrasion, a connector or coupling on the ends of the EMT can address that concern.
