Gimme dollars
I speak only from personal preference but think collecting little envelopes of local currency at the check in desk of hotels is ridiculous and prone to all sorts of problems as indicated in posts above. The allowances are pitiful as it is however at some stations one can get by a lot easier than others. For example the location of the crew hotels in BKK and SYD will allow you access to a wide choice of facilities which will allow you to choose how you budget for meals etc with your meager allowance. Places like DAC, LOS and the like have you captive in the hotel unless you are the ultra adventurer. The nature of some pairings too have a bearing on what I need in terms of food and beverage rather than what the company policy dictates therefore I always have a build up of one weird and almost unusable currency and a deficit of another in my flight bag.
There is a lot to be said for an away from base allowance as my previous company paid. At least it would be in one single currency and I could decide how and where to spend it. I am sure it would be easier to administer too.
If the newpapers are to be believed UAE has recently signed a bilateral air services agreement with Zimbabwe opening the way for Emirates to operate to Harare. Immagine a layover in a place where your allowance would be worth 25% less every day that you were there without you having to spend anything.