[methou]

It's quite popular actually, even there's an market for openwrt based mptcp routers, sometime also called SDN routers.

Say have eth, wifi(2.4+5Ghz+6Ghz) and cellular network, you may want to use them fully all together, there's a way for mptcp to shine;

or you have multiple proxies, and wants to improve reliability, you can also use mptcp.

[Slapshot_gd]

And even commercial broadband services.... In full disclosure I worked as a supplier to implement MPTCP on the CPE for BT at the launch of their service (time marker 1:48 starting point) : https://youtu.be/eMKAFWy6940