Miniclip strongly suggests upgrading any Guest account to avoid any data loss. Why?
Imagine that you finally managed to get your career started and you are getting into the game. Then, one day, you lose your phone... Everything attached to it is recovered because you saved everything, but you forgot to link your game to Facebook. Oh no, there goes all of your progress...
In situations like these, you will lose ALL the information (the level you are at, purchases, coins, items, cash, etc) related to that account. And, in most of the cases, there is no way for us to recover your Guest Account, so it's lost forever.
Before anything like this happens, lets save that Football Strike account! How?
Go to your settings and press the "Login" button. That's all it takes, now your account is linked to your Facebook account and you won't lose it.
Now you are free to enjoy the game while being safe.