Probiotics are beneficial for many health issues, which has been proven in many clinical trials (in animals as well as humans).
The problem is that not all products are of equally good quality and you need to know what you are buying.
Also, probiotics are not native bacteria to the intestinal tract, and for certain functions (e.g. synthesizing of vitamin K) you need the types that would normally be present, acidophilus and other probiotics won't fill that role.