well.. No you are not allowed to wear it. it is under special conditions that you can wear it.
1- Field ex. IF the combat is complete and IF the CO approves it. no more no less
2- during Summer camp, army cadet will get it, (For navy and air sorry, I cannot tell) and it will be complete set. (You cannot have your personal stuff on camp.)
3- for a special Activities were it will be authorized by higher ends. (CO will always have the last word.)
To my humble opinion, it cost allot of money to get a combat, for people who have not completely grown. that means that the combat uniform wont always fit. Why bother buying it when you might use it once or twice a year.