If the downtime is caused by internal SE (like maybe some SE employees "accidentally" cut all the server wire caused server down) i agree the player should incentivized.
But if the cause is external, they should not.

the reason is simple, because it's own player could do that.
Imagine this example: some player will ddos just because he cannot play at the time (weekdays at work for example) for the rewards. This player thinks it wouldn't affect his playtime at all, afterall when he comeback from works, he could just deactivate the ddos machine, the game become playable again, and he get rewards for ddos-ing the server.
Now Imagine that example happen all around the globe, with different players in different time zones, suddenly we got near zero uptime.

Maybe this is a bit extreme, but when i remember how many so called "hacker" cracking website just for bragging and "lol i hack this", i could see this will happen eventually when they implement this ddos-reward system.