There are many reasons why the opponents of the MOU say we shouldn't have one. I thought I'd discuss some of those reasons.
"We simply don't need it"
They all claim to do their due dilligence when they buy something, yet looting continues unabated in all parts of the world. Those looted items have to be going somewhere, and even a brief look at places like ebay tell us where. Despite their protests to the contrary, collectors continue to buy items with no questions asked.
"We should try programs such as the PAS first"
The problem here is that the PAS is totally voluntary. How many finds do you think are actually reported? My educated guess is only a fraction of the total finds. Again, a quick look at ebay gives us the answer. How many coins and other finds from the UK give a PAS database number? In any case whether or not there is a PAS makes no difference to the export procedure (which is what the MOU is about). All archaeological finds from Britain need an export licence to leave the country legally - just like Greece - and whether or not they have been reported to the PAS (or the Scottish Treasure Trove Unit etc.) is not any part of the export process. Also does having a PAS stop UK archaeological sites being dug over and disturbed in the search for collectable and saleable antiquities? In what way does the PAS actually protect sites?
"It's too expensive, time consuming, and would take up too much room to keep all the paperwork accociated with their coins. Time is a commodity".
Yes, time is money, but aren't there always higher costs associated with doing business the right way? If I hire an illegal immigrant to work for me it's cheaper, but then they don't have the same protections and responsibilities of a citizen or legal immigrant. But is it the right thing to do? Many companies go above and beyond the minimum of what the law allows. Not only because it's good for the corporate image, but because it's the right thing to do.
"Those countries are not doing their part to stop the looting at the source, they should focus on the looters instead of the buyers"
If you look at the source countries, many of them are impoverished, and simply don't have the funds to police the sheer volume of historical sites that literally cover their entire countries. Even those that are not considered "third world countries" like Italy, Greece, the UK, and even the US cannot possibly be everywhere at once to stop the illegal digging that is going on. So is it the fault of law enforcement, or the looters that are doing the digging?
We need MOUs because the antiquities market, including coins, has proven unwilling to police itself. Perhaps if that had happened, MOUs such as this wouldn't be necessary.