Gary Halliday,
I can substantiate the fact by saying flying IFR in a single or Multi-engined light twin is 2000 times more difficult than sitting back in a 737 with a 1st officer next to you to do the RT, pushing buttons on the autopilot and drinking freshly made coffee by the CC.
Sorry, but hand flying an IFR approach, in a small twin, single pilot is bloody difficult and the powers that be need to know that you can cope, hence the requirement to fly the test on a similar aircraft type.