I am sure you will have looked at their web site which gives the background, Mission Statement etc http://www.ovpm.org/index.php
I notice one aspect close to Els's heart - namely that, surprisingly, the UNESCO WHS logo isn't displayed anywhere that I can see on the OWHC site. Ths might be taken to imply that the organisation has less than wholehearted support from UNESCO iself. That this is not the case is demonstrated by the fact that UNESCO is a "partner" to OWHC and indeed refers to it and its jamborees on its own site. UNESCO also has a cooperation agreement with OWHC, which attends UNESCO meetings as an observer.
The word "Jamboree" takes me back to a possible reason why some cities (even Villages!!) are members and some not. No doubt it costs to join (Its site refers to "annual membership fees" but I cannot discover what these might be - there is quite a bureaucracy to support!!) but a nice little pay back will be the "fact finding visits" and annual conferences in far flung locations eg Puebla (twice!), Cuzco, Bergen and Quebec) which accompany membership. From my own country the good burghers of Edinburgh, Bath, Liverpool and Telford (for Ironbridge) have thought it "worth" joining. No doubt they will point to benefits in terms of improved management and tourism promotion etc (another logo on the City's signage and letterheading always goes down well!). My general impression is that the 215 member cities are generally of the "second ranking". They no doubt will claim that they will benefit more from such membership and the information exchange it allows compared with the largest cities. If I were a Council Tax payer in Bath where, no doubt, public lavatories are being closed, car park charges increased and rubbish collections curtailed I would be wanting to question the motives and benefits of membership - but then I am a born cynic!
PS. Riga has been considering joining and i discovered this snippet about annual costs and perceived benefits (I am impressed with the "open government" practices in Latvia!)
"As an OWHC member, Riga will have to pay the annual membership fee of 2,604 US dollars, but the organisation will include information on the Latvian capital in its internet site monthly visited by some 27,000 people all over the world. Riga will also be able to take part in all the activities by the OWHC, elect and be elected to the leading positions of the organisation."
The membership fee doesn't seem very high - whether it varies by size of city/GDP per head of the country involved I know not. There would surely have to be some variation - an African city paying the same as Bath would seem impossible.
Bath is coy about any overseas visits carried out by its employees (other than an old detail :- "A representative of the Chair of this Council attends the bi-annual OWHC Congress and in parallel the next North-European regional meeting at Santiago di Compostela") but seems to have hosted a number of visits from elsewhere
"Recent visits have included a group of Chinese planners on a study tour of historic cities, a group of Indian heritage specialists studying management plans and urban conservation, and heritage tourism planners from Romania.
These visits provide us with an opportunity to share our experience with other cities and countries, and to learn from different approaches around the world. This broadens our attitudes to heritage management and reminds us that we are managing an internationally important city."