Yes, you really need REX to build a routing topology. REX listen for all messages exchanged by the devices (OSPF,ISIS,BGP) and is able to build the exact communication paths across your infrastructure. It also stores records on how the links were at a certain time in the past, so you can troubleshoot the connection problems after they occur.
Routing table are just used in the discover connections sections of devices modeling. So you can see how devices are connected, but you only monitor L3 using traps received in Spectrum (BGP, ISIS, OSPF). You'll not be able to see the complete picture, but there is some information.