(1) Suppose two very unequal partners pair up. Assume ratings something like ACBL masterpoints: Alice with 1200 and Bob with 50 partner up against Charlie and Doug with 500 each. If Alice and Bob win, does it mean that only Bob gains rating points since Alice was better than their opponents? Or do we count them both as their average of 625 points, so that neither gains anything?
(2) How to deal with a pair that used to be much better than they are now. I like the way WBF masterpoints decay over time; something like that might be called for.
(3) How to deal with players who avoid joining the league so that their points aren't counted. I have run up against some very good players in this category at clubs. In the US you could do the same thing by joining one of ACBL/ABA and playing at the other.
(4) For that matter, a person could have multiple 'nyms on BBO. I'm sure this is against the rules but I'm not at all sure it can be caught. Even if there are 5 BBO login names on the same PC, maybe they're a family or housemates.
My feeling is that rating systems are a good idea for clubs (though (1) through (3), at least, need to be dealt with) but online games should not award points of any kind, unless they're the online forum's own private points, because it's impossible to police adequately. Only noticing cheaters if they consistently get "too good" results will catch only the stupidly greedy.
1. There are many rating systems used across my different games and this problem is essentially a solved one. There are multiple solutions, but the basic idea is very different then the point system you see with the acbl. Unlike the acbl, where the rating only goes up over time, each player has a rating that goes up when they do better then expected and down when they do worse then expected. The expectation for a pair is the average of each persons rating compared to the average of their opponents. If you want more details on this you can google lehman rating for a bridge version.
Most complaints against ratings rest the negative impact it can have on some players who become defensive of their rating. They will get much angrier about bad results and sometimes even refuse to play with or against others because of perceived imbalance in ratings.
There are some other problems, such as separate groups not being compared accurately because there isn't much cross over play and it tends to intimidate poorer players into not playing.
2. Good rating systems will always lag a little behind a player whose skill is changing, but they generally catch up.
3. Most good rating systems work off results, if the results are not done within the system then the question is moot. If they play in the system, then they will have results and be rated, doesn't matter if they join something else or not.
4. Then each login would have their own rating. I am almost 100% sure it is in fact ok to have multiple logins and I know several high profile people that do. They have a public persona and a hidden one when they want to be left alone. I am indifferent to the practice as long as they are not cheating by having multiple logins at the same time while playing.
As for your final point, that brings to mind an interesting idea, what if bbo could enable private clubs to introduce a rating system. That still leaves the main bridge club that the vast majority of people are steered towards free of it, while filling the need that is constantly being asked for. It might also allow different private clubs to experiment with different rating methods to their own determent or benefit.
As for cheating, shrug, some people cheat, both in person and online. Not much you can do about it but catch it when you can and go on with your life. No reason to let them win by spoiling it for everyone by deciding we can't do anything because some people abuse it.