Ok, well I wasn't too sure about it, as I haven't dealt with the S1 <> S2 issue myself before. It really is a shame that you have to use dedicated S1 servers to actually use the S1 content, and seems quite like a ripoff (yes, you can buy S1 content only... but nobody will play with you then).
My suggestion:
The licenses should really be more seen as different content packages (what they actually are) and not restricting S1 users from S2 server access. S1 users should be able to join all S2 servers which run a track from S1, or heck, they should be able to join every server. They just shouldn't be allowed to use locked content, so if a S1 user joins an Aston race the only thing he can do is watch. If he joins a South City server, he should be able to drive the track, but only with the unlocked cars. So if the race is for LX6/FZ50/RAC, he should be able to choose the LX6 and race along with the other cars.
Basically he can watch S2 content, but not play with it. One thing to consider is a special form of spectator-only connection to a server which takes less bandwidth but allows S1 users to watch S2-only content races without taking away one slot.
Same thing goes for S1 <> S2 <> S3. I agree, that at the moment the amount of S1 users is really way too low for the amount of work needed for this, but that is mainly because S1 content is too boring and S2 contains all the goodies. I don't know what S3 will have, but I can imagine that the gap between active S2 and S3 players will be much bigger there, so implementing this idea is a good way to not split the community and will be necessary sooner or later, IMO.