It's a concern more for cosmetic reasons and for convenience then for health or safety reasons. The saline filled implants are filled - just like it says - with saline, so if they leak, that saline will be absorbed by your body. It's the same fluid that you would get in an IV if you went to an emergency room to get hydration.
Looking at the silicone implants, the most modern generations are all filled with what they call a cohesive gel, and people often refer to these as "gummy bear-style" implants. So, it's much rarer that they will leak. Even if they were to break, the cohesivity of that silicone will maintain the shape and the silicone within the shell of the implant.

Do Breast Implants Leak?

Dr. Jason Altman explains what happens if a breast implant breaks inside the body.