I keep mine to have it next time available (that being said - I of course forgot it at home / could not find it now for two times)

Of course they would love you to register it (especially 'tourists') because then they can track your moves better.

As said before, mine took like 2 or three minutes - mainly with me asking some questions and her checking my credit card to top it up.

It always pays off to take it (the unregistered way) because then you do not need to worry about how often you will travel or not it just takes the needed amount.

Remember also you can top it up at the automat as well.