They're worth holding out for because they (significantly!) enhance your chances to craft your own Legendaries... You'll still trash plenty and maybe want to re-roll a line here and there.
Nexon have never announced an official policy on this, so they might be a one-off, or they might do these once a year around Lunar New Year... or maybe they'll surprise us and offer another one soon after the Summer Rush... in the hands of the gods (and Tin & Muet)...
But it never hurts to ask nicely... (please, please) =)
I thought those prior crafting events had been a while. Unfortunately I am sitting with 150K worth of fragments and was hoping to put them to good use but I guess I will wait and collect some more fragments.