whowhenwhy...
I don't believe that your analogy is appropriate, I'm afraid.
Yes, we use the sim to rehearse all sorts of drills, in an environment where our actions don't impinge on the safety of real passengers and aircraft. Sometimes, we train in empty aircraft, but in a careful and measured manner.
We don't, of course, simulate emergencies on scheduled sectors with passengers aboard - and as 121.5 is a real, not simulated, distress channel, I believe that it should not be clogged up with pleasure pilots doing their training.
Lost comms have a new and horrible connotation since the WTC attack, and the fall-back process of contact on 121.5 mitigates against someone mis-percieving a simple loss of comms event as something a very great deal more sinister, and even more horribly, starting to take action.
There are two principle problems with training use of 121.5: First, that the 'chatter' will cause professional crews to de-select 121.5, and then leave a crew in a lost comms situation from which there is no short-term escape (often, lost comms goes un-noticed and un-known by one party for a considerable length of time). Second, that a crew monitoring 121.5 might be sufficiently distracted by chatter to commit an error.
I don't condone the use of 121.5 for airline chat either, though it's worth considering that, in using box 2 or 3 for 121.5, one no llonger has the 'company' frequency available.