Hmm on my development machine MySQL is already there, when Maria DB becomes an option I'll switch. Probably same with my Hosting.
If they are technically the same, the cost for me currently is the same ($0), and the experience is the same, there is not immediate justification. Now when Maria adds some nifty new feature (schema-less data storage, intelligent joined results, or some other cool, thing that takes the bite out of massive SQL queries) then it becomes compelling because it isn't same-old/same-old.
I'm sure the same goes for a lot of others; additionally I am comforted that Maria exists and is being developed, and if my situation changes that will be the direction I go.