The ATPL schools make their students wear uniforms "to get them used to the professional environment" but in normal GA in Europe one just looks silly.
Even down in Greece I cannot imagine it will be of any use. In some cases a passenger could pretend to be "crew" and avoid the 10-20 euro passenger tax but IME they are likely to ask to see his/her license...
In Spain, in years past, a uniform would get you tax free avgas at certain airports, and this was really worth it (a ~ 30% saving) but I don't think it works anymore. You need to produce a piece of paper with "AOC" on it at the pump for this to work. You also need to produce a VAT number, and this does apparently get checked by somebody back at the office.
IMHO, a uniform is also damn uncomfortable to wear in a warm cockpit for a long period.