Sure it is optional. But try to build a solid deck from the resources you get for GGs and daily rewards only. How long will it take you if a competitive deck from gwentdb is about 5300 - 5400 scraps... And, even if you spend - I have to warn all others intended to do so... I did, personaly. In fact it is not worth it. If you are after legendaries, I received about 3 for each 60 kegs I bought. In the last month I may have made such a purchase about 5 times. As I was about to test some top rated decks from gwentdb to have at least one good for each faction. In the proces, I lost everything I bought, because most of the decks I tested proved to be - let's say it decently - highly incompetitive. So why store cards you don't actualy play? So I milled the majority of them again, in search for good decks. In the end, I was able to find 5 good decks, one for each faction, but allmost my entire collection built since the start of open beta is gone. So to the others - don't mill your cards for the sake of getting those that you need. Better forfeit the deck you would like to play so much, yet you lack the resources, and slowly, but surely, continue to build your collection - 1 gold and silver each, 3 bronzes each. Gradualy, but in long term, you will be able to play every possible deck this way.

Addictive as you say it might be, but if your real world resources are limited, like my case, than this will keep a good control of you :-) Fortunately enough... As for losing - no reason to quit. I lose too, actually more than I win. Yet I could become rank 19 even so, which is not that bad at all. Be stubborn enough, and you will see you will manage too, or even do better. Best of luck to you at that (y)