While I agree Barcelona has a pick pocket problem, in my opinion this is scaremongering and overkill.
It is ALWAYS better to book hotels direct and not through a third party or travel agent. The third party will own the booking not you the guest and if there are issues or you need to cancel or extend a stay, the hotel will not be able to that much for you. Whereas booking direct with the hotel you will not only get a better rate but the hotel will be much more easy to deal with if there are issues and they may also have perks or something extra not available if you book through a third party or travel agent.
There is absolutely no need to use a car service called by the hotel. Barcelona like most European cities has a very efficient, safe and regular public transport system, which is used by the vast majority of locals and visitors to the city. If you really have to use a taxi, use the FREE NOW App to call a metered taxi. Getting the hotel to call you a car service is tourist trap and you will end up paying a lot more than just getting a regular metered taxi.