Let me first state that I have never put much value on the whole short vs long action debate: a couple ounces weight savings, a negligent difference in bolt throw and OAL, and a "theoretical" advantage in action stiffness have never been reasons enough for me to think, "oh golly, I need to replace my 6.5x55 with a .260 ASAP".
Most importantly, I've largely turned away from short action chamberings due to one very important disadvantage and that is their limited overall cartridge length. I have a preference for heavy for caliber bullets and to seat a 250 grain cup/core (never mind the longer for weight copper bullets) in a .358 Win results in eating up way too much powder space.
With the advent of copper bullets (and their longer OAL per/weight) and their rising popularity, I think that the medium and long action cartridges will have a clear advantage when it comes to the flexibility of the cartridge (usable range of bullet weights). While I disagree with it, some places are even legislating the use of all-copper bullets to make it mandatory.
To be fair, the expanding monometal bullets are considerably better than traditional cup/core so a 130gr .264 TSX will not only outperform a 160gr round nose in velocity/trajectory, but will also have superior penetration (see .264 CGN bullet tests), and this is likely all a person would need to hunt all ungulate species. But we still need to compare apples to apples and consider applications.
The option to load up 7mm 175 gr. TSX with superior BC and SD at 2400fps is a great motivator to consider a 7x57mm over the 7mm-08, whose OAL limits use to 140gr cup/core. Same goes for any .308 based cartridges and their medium/long action "ballistic twins".
I hunt heavier game and thus I am partial to heavy for caliber bullets based on my experience alone. Lighter and faster bullets do not have a monopoly on external ballistics. BC and a bullets retained energy count for a lot and this is the domain of heavy for caliber. There are also clear terminal advantages in going heavy for caliber. I foresee with these long for weight bullets that the previously understood advantages (particularly within the reloading community) of running heavier bullets in those medium and long action cartridges will be even more apparent.
I am NOT disparaging the .308 based cartridges. I think they are great albeit not as flexible as their medium and long actions counterparts. For many (the majority?) of people, this doesn't matter because they hunt with only one bullet weight. The problem, however, is when you want that one bullet to be heavier for caliber. With a .308 based cartridge you're pretty much SOL unless you want to eat up case capacity.
What ye say?