When is the right time to use MariaDB instead of MySQL, and Why? What we earn and what we lost with this migration?

What should I expect as drawbacks after the migration?

Is it really unnecessary to change the applications in any situation?
I think now is the time to play with MariaDB (just like now is the time to play with anything like this). Personally, I would hold off until I know a few more devs who have deployed MariaDB onto a live customer site before doing it myself.

Some situations could call for a change of application but something as serious as changing your database engine is not something that should be considered lightly.