I'm not sure if mine was truly "medically necessary"--pushing in a different position or massage might have resulted in a different situation. I had a band of "fibrous tissue" that was restricting dd's head from coming any further (you could see the distinct ring around her head for days afterward). I actually had had issues with this "fibrous band" during, uh, personal relations (

) and feeling as though it were tearing slightly each time, so I knew exactly what my midwife was talking about when she mentioned it. Do I think it would have been possible to birth dd without the episiotomy, sure! I probably would have torn given the position I was in, but I do think it's possible other measures might have prevented it from being such an issue...but I do feel, in my case, it was warranted at least, if not "medically necessary." I don't feel it will be an issue at all with the next birth because of the episiotomy (and it's improved our "relations" as well because that super tight band is no longer a problem), but the episiotomy isn't even on my list of regrets from the delivery and aftermath. Had it been an OB I didn't know or trust, I'm sure it might have turned out much worse! My midwife knew I didn't want one, doesn't do them as standard practice, and did as little damage as she possibly could.
We did end up with a slight shoulder dystocia but that was AFTER the episiotomy!