I have maxed all my common cards - but probably this was not so smart from an investor's view. there will be tournaments for gold foil cards only, but I do not think that those will be profitable if it was your only reason to buy the cards. They are said to have a price pool of 1000 SBD per month. so assuming that the winner will not get all and that you will have to spend a lot of money to win those, I do not think it is a good decision why you should buy them.

Will they really lose value if you use them? I don't know as this is an irrational thing, but I could imagine that a collector would want them fresh just as they had been created. I could be completely wrong on this, but clearly I would not combine them as of now as this cannot be reversed and you could still combine them any time you want if it becomes favourable.