While the 50mm can be used as a loupe, here's a couple of reasons why you woudn't want to use it as one.
1. Can't view the entire slide.
2. Sharpness isn't there - 50mm lens weren't corrected to focus images at such near distances.
3. Have to stick your head unbearably close to the rear element...
4. More fragile.
A good 4x Rodenstock / Schneider costs about the same as a 50mm. The 50mm lens has a magnification of about 5x+