I've been a jpa user on deployment and a wider use of mod equivalent systems. I fully get that the idea of why some people dont like having to get receipts, but its just part of life.
Ultimately jpa works reasonably well and its teething troubles are long gone (which by sounds of things is where some experiences here come from). Just because at one end giving cash out to user is easier doesnt mean the admin burden is reduced, it just shifts where it happens in a more visible way.
In a time of tight budgets i'd rsther reumburse actuals than just keep paying daily allowances greater than what is needed if that helps protect the front line. The military dont have an exclusive claim to deploying people at distance around the world and having to do admin. Other large groups manage perfectly well, so i just genuinelt cant see why the military needs to be different in this case?