The friendly games are just to help the coaches assess their team's performance and progress so far and it is not a competition that the fans should whine about a lose. Soon the world cup will be over and every player will be returning back to their clubs to resume the season. Arsenal also had a game against Lyon which ended in a 3 nil win in Arsenal's favour. The boys put up a good show and dominated the game so well.
I agree, it is not right to care too much about friendly matches for this reason. These are the matches where the team does not play with the team it should be, the development of the players is observed and the coaches try something new.
Otherwise, of course, a team like Manchester United would not have lost this match. At the moment, neither team is actually playing as it should, after the world cup is over, every team will form a roster as it should
Another important part is that how many players United has that was in the world cup and how many did Cadiz had? This means that Cadiz probably played with their starting 11, I do not know them very well but that's what I am guessing, whereas United played with players that weren't picked up by world cup because all other good ones were in the world cup.
That is why this game meant nothing, if it ever did then we would have seen all their good players there as well and since they were in the world cup this wasn't even United on their full form. I am guessing that this was more about keeping in shape than anything else and that is why it didn't matter.