First of all, there is nothing wrong with apps like SnoopSnitch in general. We can be glad that there are people who are concerned with this matter and build such apps. Nevertheless, you have to understand that such apps are pretty ineffective in the vast majority of cases.
SnoopSnitch, for example, only works on 2G and 3G networks if your phone is rooted and if a very special chip is installed on the mainboard of your device. You need to understand that communication with the mobile network is a completely opaque black box for your operating system. Your operating system and your apps are not able to control or monitor communication with a radio tower (base station) in detail. This means that the wireless network can communicate with the chip on your device without your smartphone being aware of it.
This is due to proprietary, commercial hardware that is not open source. This is also the reason why you can be roughly located by silent SMS (stealth ping). The radio chip in your phone registers this, but does not report it to your operating system. Only a few chips have interfaces that allow the operating system to monitor them. SnoopSnitch is only available for this purpose.
Even if you have such a phone: Mobile radio consists of so many different standards and technologies that most apps show completely different results.
One possible sensible defense is an anonymous sim card. This can help you more effectively in most cases, as it simply hides the target of the attack.
If you are seriously interested in this topic, you can also support the "Rayhunter" project of the Electronic Frontier Foundation (EFF). This project collects data on possible IMSI catchers in an attempt to better understand the problem.