In addition to the lack of Lagrange points, another unrealistic aspect to the patched conics approximation is the stability of orbits. In reality, if you were not at a Lagrange point then your orbit would not be stable. For example you couldn't be in a stable kerbolcentric orbit just outside the SOI of kerbin, kerbin would pull you back in eventually. For example, the Apollo 12 third stage recently returned to Earth orbit (and was mistaken for an asteroid initially): http://neo.jpl.nasa.gov/news/news134.html