The strapline should offer people something that they don’t already have. Many people will perceive that “everyone” already has “secure” access to Facebook, or Google. Why should they feel drawn to yet another service that (as far as they may understand) offers nothing more?
“Privacy, Security, Freedom” answers the “Why” in a concise, positive way. It would be a missed opportunity to merely repeat the name of the network when the name by itself will convey little meaning to the uninitiated.
The word “safe” means very different things to different people. (And, as has been discussed in the past, a likely source of propaganda against the Network will be from people/groups who claim that the network has failed in preventing “unsafe” uses and behaviors.)
So, what does “safe” mean? “Privacy, Security, Freedom” defines that. It also establishes the core values of the Network in a way that can not so easily be mischaracterized and co-opted later (be it years or decades in the future). I think that this point is very important.
Although one could argue that the “vision” of the Network is already established as “Privacy, Security, Freedom”—where will most potential, or even actual, users of the Network be made explicitly aware of this vision if not through the strapline? What use to the public is the vision if they are not made aware of it?
I think it is important to define what “safe” is. Simply repeating the name, or referring to an undefined “access” does not do that. “Privacy, Security, Freedom” answers the core “why” question, and defines the values of the Network in a way that can preserve them far into the future.