Why should a machine gun (whatever you care to call it) have a semi-automatic or select fire capability? Supposedly the policy was for the Bren gun to be used as a semi-automatic weapon "to disguise from the enemy that a machine gun is being used against them." Are machine gunners really trained to try to achieve single shots on a full-auto only weapon?
For the first question, if you mean 'machine gun' like a full auto M4, then the select fire is there for options, like mentioned above. I'm not going to be using burst fire at near the weapon's max effective range. Yes, I could quickly let off the trigger with full auto as to only shoot one round, but that makes for much more difficult precision shots. No idea why they would want to disguise a machine gun, as it's a great psychological weapon. Does make you a priority target though. Double edged sword; you're much more dangerous, but more important to be targeted earlier.
For belt fed weapons, single shots aren't too hard to achieve. The 240B is quite easy to fire in single shots, but it is basically used to determine where your shots hit in relation to your sights. I can see that being used for a vehicle gunner to get used to the weapon so they can compensate. The slower the rate of fire, the easier it is to control how many rounds you send down range, of course. For belt fed weapons, not really needed, in my opinion. It would be neat to see a selector switch for rate of fire beyond adjusting the gas feed though.