So far, we've seen a 100% success rate for this new process in rushed situations/combat-type environments. If the process really is so dangerous, Spock Prime is kind of a jerk. "Here is your formula for transwarp beaming Mr. Scott. Uh, it has a high mortality rate, and nobody in their right mind would do this in ordinary circumstances, just so you know." We don't get this speech in the film, however, which indicates that Spock Prime feels it is safe enough that he doesn't feel the need to say anything. Your point is reasonable, it is something which could be written into the text, but it hasn't been written in yet. What has been written in is that the trip from Earth to Chronos is just a transporter jump away. And let's suppose, for the sake of argument, that they do write it into the text. Even here, however, it would still seem reasonable to transport anything but a person across these distances (e.g., food, medicine, supplies). The Enterprise should no longer have "pack mule" plots where she has to move medicine or food around (a common source of drama in Trek - a time sensitive mission vs. pressing exigent circumstance). For the time being, you could also argue that this is super-secret technology which they don't want to share which would still necessitate that Starships go through the motions of moving stuff around. The question then becomes how long this remains a secret (military secrets are the most fleeting of all).