Short magnums...
Paco Rocks--I happened to get a deal on a Savage in .300 WSM. What I was looking for was a .30-'06 on steroids, and that exactly describes the .300WSM. So I'm happy. It is a little slower than the .300 Win Mag, and burns a little less powder per round, and of course, would recoil a little less if fired from an identical rifle. Also, it isn't good with bullets of more than about 180 grains. But it suits my needs fine.
Now--Is there room for all the new short magnums being introduced? I think not. It would appear that the RSAUM's are dead in the water--Winchester got the jump on them. I also think that with a .270 WSM, a 7mm WSM, a .300 WSM, and a .325 WSM, the field is too crowded, and some of them will die off.
The death knell for the RSAUM's was when Remington started chambering its rifles for WSM's--You didn't see Winchester chambering its offerings for the RSAUM's!! (Although Win was starting to go under about then, which complicates the calculation!)
This new Ruger Compact round?? It would appear that Ruger is too late jumping on the bandwagon. I'm willing to predict that it won't be able to crowd its way in, unless something unusual happens, like the Ruger round taking Camp Perry by storm, for example. Which I don't think it's about to do.
Now, to your original question: Do shortened cases make a significant difference? My answer: Not that you'd notice when firing them. Rifles last for so long--generations, usually--that the manufacturers have to keep bringing out new stuff to keep selling rifles.
One or 2 of the new short magnums will become something of a "standard cartridge," like the .243 Win, .30-30, .30-'06, .308 Win, 7mm Rem Mag, .375 H&H, et al. The others will, in 10 years, I predict, become rarities like the .303 Savage, the .350 Rem Mag, and a host of others. Will ammo for them disappear? Probably not, but it won't be available at every Mom & Pop store, either.
If you want one, I'd say get one. Their ballistics are nice enough, and if you handload, then ammo availability becomes a secondary consideration.