The wiki documentation at http://wiki.mikrotik.com/wiki/Manual:IP/Route states:
Routes with interface name as the value of gateway are not used for nexthop lookup. If route has both interface nexthops and active IP address nexthops, then interface nexthops are ignored.
I’ve tested routes with a single gateway in form “IP%interface” and discovered that such routes are not used for nexthop lookup too. My question is what is the reason for that?