Should be easy to implement and will be useful, for example enabling ADSL/3G interface and switching routes if 8.8.8.8 is not pingable from main uplink.
This is what you seem to be looking for: http://wiki.mikrotik.com/wiki/Advanced_Routing_Failover_without_Scripting
Non exactly that. Example:
We have fallback ADSL provider who charges us based on if we connecting to them in certain day or not. If not, charge will be small, if yes - pretty high. If i use routing failover, i need to keep interface up all time, meaning i should pay for fallback channel more than for primary 
And there is no way in routing fallback to notify me that primary uplink went down. Or i don’t know how.
I would just create a /32 route for the destination you intend to ping, pointing to your main gateway.
8.8.4.4 would be better for that, since a lot of people may actually use 8.8.8.8 as primary dns.
Thanks, good idea. Gotta find some useless IPs for it. But gw option will be way easier.