I went on the RS Spain/Portugal tour in July which began in Barcelona, on to Madrid, Toledo, Granada, Ronda, Seville and Lisbon, I traveled to Paris on my own after the tour ended. I wore the RS Hidden Pocket which held my passport, ATM card and credit cards and carried the Pacsafe Pouch-safe - sort of wallet on a strap across my body as well as carrying a Longchamps Pliage tote bag for tour books and water bottle. If anyone had tried to grab it, they wouldn't have gotten much.
I was very nervous when I arrived on my own in Barcelona after hearing of all the crimes against tourists, especially in the La Rambla area. I didn't have any problem at all, I was very aware at all times, keeping my hand on my bag. In all the other places, I didn't experience anything either. I did see a police presence in the busier areas of the cities so I felt safer.
However, one man on our tour (a big guy from Texas) did have his passport and small camera in his shorts pocket and he had those stolen...which created some extra work for our guide and him to go to the police to file a report, go to the embassy to start the replacement process and for him to return to Barcelona after the tour ended to pick up the replacement passport. Fortunately his flight home was from BCN, otherwise he may have had to make some expensive changes to his travel plans.
I know there are people who say that they never wear money belts, they carry backpacks with all their important items inside and they have never had any problems - they are very fortunate. I have also been on RS tours where people have been pickpocketed...usually on the Metros, but sometimes on the street - not victims of violent crime but still had a large amount of money or their cameras taken.
You don't need to be paranoid or fearful, just be aware. With the declining economies around the world, thieves are taking advantage of careless tourists. Enjoy your visit to Spain - I can't wait to return after my brief taste.