Our school d/n have an EM clerkship in third yr either. I am doing two away electives in EM. Neither of them are my sub-I, just electives. Here is the reason. If you are going to do an EM rotation at your home institution, then it would probably be beneficial to do a sub-I rather than an elective. I say that because at our program many people take EM b/c the hours are easy, so they take it during step 2 season to study or interview season to leave without being down-graded. That being said, if you just take an elective they might not consider you to be as serious about EM as you might really be. If you do an away rotation in a given discipline (EM included) it is assumed that you are "serious" about the rotation thus making a sub-i unnecessary.
Is it necessary to do more than one rotation in EM? That all depends on how confident you feel in your competitive factor. Me personally, I would like to be more competitive on paper, so I am going to show my ugly mug at the programs I am interested in to help my application process. If you are decent on paper and are at an institution with an EM Residency (and you can get a SLOR from them), then multiple away rotations are more optional than necessary.
Hope this helps.
