There is a further issue (minor, still …) you are “burning” a DNS server for each recursive instance.
Although most examples you can find are using 8.8.8.8, many people will use it as a hardcoded DNS server, so in real setups one should use for recursive less common but still highly reliable ones.
There are similar, but different ways, here Is another one, I believe easier to understand:
http://forum.mikrotik.com/t/simpler-failover-for-two-gateways-i-found-working/169108/1