Well, its a lot easier to use TiFlash than replicating your OLTP data to a read-optimized database (whatever it is).

1. Add TiFlash nodes to your TiDB cluster (you might need to config some ports and ip address just like other databases and then start the nodes)

2. ALTER TABLE your_table set TiFlash replica 1;

That's it. You won't need all data in memory and replicating details are taken care of silently (schema change, fault-tolerance, load-balance, consistency and etc).

I believe heterogeneous replicating even with minutes latency tolerance will not be as easy as above :)

PS: the good part of keep TP/AP consistent is that user can use them in a single application without any surprise: it's just a different forms of a single logical data.

PS2: I'm a dev of PingCAP.