Pictured on the right is my current collection of plugs and chargers - North American-type plug for (locked) Canadian cell phone, UK type plug for UK phone, charger with Europlug for South African phone (not ideal, but the phone was only $15), plus a UK and South African adaptor thrown in for artistic effect.The theory of optimal currency areas should apply to electric current and plugs, too. Multiple plug types, like multiple currencies, impose costs on travellers, and are a barrier to trade. This argues for widespread plug standardization. So why does it face resistance? Is there a potential difference between currents and currency?
The argument against large currency areas is flexibility - governments should be able to use policy to respond to changed circumstances. I don't see this argument applying as strongly to electrical shocks as economic ones.
A more persuasive explanation is that history matters. Once a set of electrical standards becomes hard-wired, it is difficult to change.
Also, some people benefit from barriers to trade. In the picture above, it's not just the wall plugs that are different for each charger, it's the part that attaches to the cell phone as well. Standardization facilitates competition. For example, with standard chargers, a consumer who loses his or her charger can buy a replacement from any company - or just use an old charger to replace the lost one. Good for consumers - maybe not be so good for people who manufacture and sell chargers.
So I don't expect to see the emergence of optimal currentsy areas any time soon.