There is thread here that might have some background:
http://forum.mikrotik.com/t/routing-rule-use-cases/163178/1
Depending on how you do it, you still often need some mangle rule to mark-connection/mark-routing. You just can use /routing/rules to avoid some mangles. e.g. “input” traffic should still go out same interface it came in on & failover cases, which cannot be covered with multiple WAN using policy routing alone.
So I kinda view policy routing as quick way to adjust the routing table used by a client or VLAN/subnet. Let mangle do the marking based on what table the /routing/rule selected on output/forward. This is in opposition to letting something like PCC or other specific FW rules do the route table picking in mangle.