Practice Pans - are there any genuine reasons not to use them?
I presume you are from UK since it is the only country I know of that uses practice pans. 

Basically, ICAO (ICAO Annex 10, Volume V, § 4.1.3.1.1) states that the emergency frequency 121.5 MHz (sometimes called "guard") shall be used only for genuine emergencies. So typically any other use is considered misuse. 

However, countries can file deviations from this rule and publish these deviations in their AIP. One such example is the UK, where 121.5 can be used for practice PAN calls to ensure pilot familiarity with the process.

Pilots not familiar with the situation in the UK sometimes frown upon this use, and as you mention, jump in with the dreaded "transmitting on guard" sentence. But those doing that seem to have the same reaction when a real emergency is going on. 

If you are flying in the UK and want to practice an emergency situation you are encouraged to do so by the D&amp;D cells. But remember to ask permission first:


  A/C > London Centre, callsign, request Practice Pan


If they have the time to accommodate you, the reply will be something like


  London > callsign, Practice Pan acknowledged, continue when ready 


And remember, it is not only a practice for you, but also for controllers. There is a benefit for everyone flying around.