I think they’re a great idea. Particularly since PIS started saying ‘You’re in coach X of Y’. I think it’s a great way to help capacity.
Also, bit of trivia, I remember reading that once there was a service from somewhere on what is now Southern territory that split towards London, with a portion to London Victoria and London Bridge. Apparently it was the only service to divide towards London and attach on the way out. Great idea, but I totally get that you wouldn’t a train failing to attach at East Croydon during the peak.
Normally, as you perhaps imply, splitting / attaching trains in the Southern area are about adding the maximum number of coaches to trains in the London zones (not just in the peaks, but especially at those times) whilst still serving the maximum number of outer stations.
There are few Southern / South Central ish area branch lines which are so squeezed for capacity and connection opportunities that they can only accommodate one train heading north, with a divide to serve two separate destinations. In fact, there aren't really any, once stock types and so on are taken into account.
Therefore, the only reason that would need to be done would probably be to balance stock between termini. Running "contra-peak" trains to get stock into termini, either out of service or with limited stops, is now the preferred alternative to move trains around.
Aside from all that... you can only split or attach trains in Platforms 4 and 5 at East Croydon anyway, as they are the only ones with permissive working. That rather limits the success of the plan if platform workings go wrong. This is probably one reason that this isn't done at this location any more!