As others have said, the 50 SC card isn't worth it. The biggest gain on stamp card is going from 50->100 due to the extra 5 SC plus all other rewards being upgraded/increased. If you're not going to hit that, I wouldn't bother.
Exception: Due to the lack of events, using SC for silver farming isn't a terrible idea, if you don't trust the IIRC 6% silver base summon rate lol (or don't have the gold to spam that). I would highly recommend holding on to many of your crystals though, and use only natural regeneration to run event maps for the silvers you want, only using SC if Sunday/Monday rolls around and you still don't have enough of the silvers you could get from the event maps and still need.