There is one thing that the RS can't do.
If A & B are doing 3rd party peering via the RS, the fact that the
A/RS peering is up & working and that the B/RS peering is up &
working unfortunately does not tell you if A & B can exchange
If A & B are peering directly, then the fact that the peering is
up also tells you that they can exchange packets.
Luckily this sort of breakage does not happen very often.
Unluckily, if it does break, if can be really hard to diagnose.
This could happen at a non-broadcast media NAP such as ATM nap when the
PVCs between RS-A & RS-B are up but the PVC between A-B is down. But it's
less likely to happen on a broadcast media NAP.
It'd be ideal that the RS is injected with some intellegence to detect the
fact that A can no long talk to B directly and thus stop passing routes between
A & B. This will avoid the problem. But as you pointed out, it rarely
happens so it may not worth the effort.
By the way, if we flip this example around a bit: if A & B peer with the RS
in addition to peer directly with each other and the bgp session between A & B
broke for some reason and the connection is still there (i.e. they do not have
bgp session but still can exchange packets), A & B would benifit from the
(speaking for myself)