I did not notice the change until reading this post, but on my CRS305, now running 7.7beta4, I have 2 cores as well.
This looks similar like on IBM mainframes, more cores can be enabled by SW. Except: IBM will ask you something in reverse....
OR maybe this is the remedy for the unavailable RB5009s, for every further quarter missed, we get one additional core...
Bad jokes aside:
I did not experience any issues until now, but I only have a simple "switch with VLANs" config. No routing, nothing else fancy, almost everything left at default.
And I ran that switch only for a few hours, but copying a lot of files to a NAS over it.