Switches use L4 ports to implement flow-switching. When multiple paths for a packet are available, it is convenient (it leads to less re-ordering) if the related packets from the same flow all choose the same path.
It's not just convenience for anycast addresses, with multiple pathes and anycast if your round robin packets across different routes, you may end up receiving them in two different locations (ok for UDP, but TCP broken completely).
Anycast is widely deployed now and imho ECMP support is now considered "standard".
I'm not sure what are other alternative solution to ECMP that don't require looking into TCP headers.