I followed this thread for quite a while now, and well, I don't think that such a rating system would change that much at all.
My team as well as myself are driving in multiple series at the same time, when the races of the different series take place on different days of the week.
What you just have to keep in mind is that probably 70% or 80% of all the servers in the lobby which are password protected are training servers for a specific league (notice their domains in the server name, for example). Those servers are usually not used for spontaneous races. You usually won't find 10+ people on there because people tend to train on different times, besides that, switching to the race session is usually prohibited, since other people who want to join then to start some training can't do that anymore.
I believe that most of the very fast drivers are driving in leagues. You can argue about that, but I believe it's simply a fact. We know quite a lot teams with great drivers who are quite succesful as well, and all of those guys are never driving on a public server. I for instance was driving on a public server probably like 3 years ago for the last time. It's boring because of the anonymity and the lack of good drivers. If you really want some competition, for me, the only solution is joining a league. But you got to watch out for ones where you won't be at least on the podium every time you race, and that's the more tricky part which of course is time consuming, agreed. But if you've been in the scene for quite a while (well, at least nation-wide, that is) you will get to know the names of people and whole teams who will make you work hard to earn some points in a championship. And when there's a new championship in one of the major leagues and a lot of those guys are taking part in it - join in too.
I'm usually driving in endurance series/events based on the Enduracer mod atm, but a V8factor unleashed series with a lot of competition became the most intense lately, where you would fight wheel to wheel for dozens of laps with some of the well known drivers around here. In this series, I was so happy to be actually driving somewhere around P2 or P3 when finishing in 5th position in the end due to speeding in the pit lane, fighting a lot of good drivers along the way where you can be sure you can make some great maneuvers because you simply know that they're always in 100% control of their car.
I believe that this is something you can't get in a public race, even with such a system. A simple value which should determine safety and/or speed won't tell you if someone is really a great driver or a wrecker. You don't know where he's coming from. And this is probably one of the reasons why most of the faster drivers are never seen on public servers. They need a constant environment to see if they're doing well. If you become stronger and stronger with every race it's probably you since the whole pack is more or less constant (since it consists of the same drivers). You can compare yourselves to each other, especially when knowing each other for quite a while. And when things are starting to turn from bad to worse, it's probably based on how much you spent training as well.
I know that a lot of people do not have the time to drive in leagues or that it's simply not giving them what they expect, but yeah, once you've found the right one (which can be time consuming, yes) you will have fun AND great competition for ages. A rating system won't change that for me, and I'm sure it won't for a lot of others as well.
My 2 cents.