I've said my views on this many times and ways, but the important argument is, you learn bad habits. A woman has so much more capacity than is allowed or seen in L&D, and it's hard for any human being, even a midwife, to believe in possibilities when it is consistently re-enforced upon them that the opposite is true.
And there is really nothing you need to know about L&D nursing that you cannot learn on the job as a midwife, if you end up working in a hospital, which many of us do. Nobody ever suggests that doctors learn to be L&D nurses first, and they have to work just as closely with them, if not even more - ie, PIH pts, c-sections, pitocin management (which midwives may deal with on some levels, but much less). Midwifery isnt a step-up from L&D nursing anymore than an OB is.