It's all very well criticising the more important members of the Royal family for the choices they make but the reality is that there are few choices available to them. They have to try to make the most out of every opportunity that they can within the rules that constantly constrain them.
They may like a boozy night out (who doesn't?) but both William and Harry are capable and adventurous young men desperate to earn the publics respect and the AAC are crying out for people to volunteer. It is very demoralising for Harry to not be able to fight alongside other members of his Regiment (who by the way are Cavalry - not Infantry as someone said earlier). They both still want to have a front line role so why begrudge them using this opportunity.
It may be a bit harder for their instructor to chop them if they are struggling but if they do turn out to be hopeless I believe they would be failed.
Those posters with chips on your shoulders about this are full of impractical, left wing ideology.
I say - in the wet society that exists in this country today, thank god some public figure heads are willing to join up, get stuck-in in the front line and take some risk.