I think there's something like that in the US already, but I don't know how much use it is put to. Here's my take on it.
We aren't particularly frequent marina visitors, but we do stop into one occasionally. Here on the East Coast US I will use our cell phone to call ahead to a marina on the day we expect to stop in there to see if they have room for us that night. I hear the large power yachts on VHF confirming that they have a reservation for that night so it seems that many, if not most, boats along this coast use their cell phones to make marina reservations.
I'm not so sure that on line reservations would work as well or be as popular. First, for many of us our plans are made as we travel; weather conditions and how much distance we are able to make towards our final destination when we are just day-tripping along. Even when we know we're going to want to stop at a marina, we can be several days too early or late when we first make our plans.
For vacationers with only a week or two on their boat, though, I think that perhaps there would be a greater usage since their plans are less flexible than a full-time (or even part-time, seasonal) cruiser.
I think that if it gave the prices for a slip online for each marina subscribing to such a service, even by those of us who have a hard time knowing what we are going to be doing tomorrow that it might be fairly well-used, though.
Pretty sure that the average cruiser would shop prices very carefully, and if the marina didn't live up to its promise you might hear some loud complaints.
As for the rest of the world? I know nothing about marinas in the Med, so can't comment. For the Baltic, we stopped in a marina practically every night, and they were all "first-come, first-served" and in most cases, "we'll take everybody who shows up, even if they have to raft up three- or four-boats deep." No reservations. True equal-opportunity, non-discriminating marina services (can you tell that I liked the idea? Although I think that it is a bit difficult for an American to understand the idea.)
Just my opinion, of course.