The theft deterrence afforded by a removable faceplate is mentioned in many patent applications. For example:
Typically, the purpose of the removable faceplate is to prevent theft of the base unit, and associated damage to the vehicle, by minimizing the economic gain that is realizable by the theft. That is, because the resale value of a base unit without a faceplate is minimal, a thief has little incentive to break into the vehicle to steal the base unit.1
Theft deterrence can be found as part of the actual claims of several patents on removable faceplates. For example, see Claim 10 of EP1992076: "The method for listening of Claim 1 wherein said removing the car security face plate assembly provides a theft protection for the car stereo system".3 If this invention did not actually deliver the promised result (theft protection), then this claim would be invalid.
I haven't found a study that presents statistics on thefts of stereos with removable faceplates vs thefts of stereos without removable faceplates.
What I did find was that "FBI reports indicate that car stereo theft decreased by more than 50 per cent between 1994 and 2009",2 and "the removable faceplates on many units also help to discourage theft."2
Police reported that the introduction of a detachable faceplate "effectively prevented theft".4
Detective Sgt. Gary Jackson (Vandallia, OH) said, "both of these features are good theft preventive measures only if the owners use them", when talking about removable stereos and removable faceplates.5
Professor Andrew Karmen said that the detachable faceplates on cars have made a big difference.6
1. Epstein, Michael and Pasieka, Michael. A multi-function removable faceplate for an entertainment system, such as a broadcast receiver. WO2001058062 A2, 2001
2. Will, Joanne. How to reset your car stereo code. The Globe and Mail. October 11, 2012
3. Hovden, Gunnar. A stereo security face plate assembly and method. EP 1992076 A2, 2007
4. Thölke, Jürg M., Hultinka, Erik Jan, and Robbenb, Henry S. J. Launching new product features: a multiple case examination. Journal of Product Innovation Management. 18(1), January 2001.
5. Babcock, Jim. THEFT FROM VEHICLES PREVENTABLE. Dayton Daily News. November 14, 2002.
6. Friedman, Naparstek, and Taussig-Rubbo. Audible Car Alarms Don't Work. Transportation Alternatives. 2003.