Before L3 switches if you had a pair of core switches they were obviously L2.
This meant all your distribution devices eg. routers would need to be on a common subnet so they could peer with each other. So you often had multiple L3 devices on the same subnet.
Nowadays with L3 switches this is less of an issue but it is still quite common to have multiple L3 devices on a single subnet eg. a pair of L3 switches peering with a firewall for example, or a pair of L3 switches peering with a router that only has one spare interface etc.
It really depends on the requirements of the network but don't make the assumption that routers always use P2P links for everything because they don't.
Jon
