"i dont see the big deal" - First, I'll give you some history, you have to keep in mind what the 6.8 Rem SPC was created for. It was designed in conjunction with some special forces individuals who desired an "uprgade" to the 5.56x45, or as it is commercially known as the .223 Remington. The intent of the 6.8 was to improve lethality from short barreled m-4 rifles on close range targets, and increase lethality on long range targets with a longer barreled m-16. You see, the .223 does well out to 400 meters when shot out of an m-16 that has a 20" barrel, but you shorten the barrel to 16", or even in some cases 10.5", and that little bullet never gets up to the desired velocity that made it effective out of 20" barreled rifles. The 6.8 improves the lethality over the 5.56 in both short barrel, close quarters rifles and long range, medium length barrels.
Second, the 6.8 was designed to operate out of an m-16 type weapon, and therefore needed an overall length to be as short as the 5.56x45. The .243 Win, as you know, was based off of the .308 Win (which is the civilianized version of the 7.62x51 NATO). But neither can be fired through the ar-15/m-16 action. They are just too long.
Third, you suggested that it had inferior ballistics to the .243 and yet proponents of the 6.8 infer that it has exceptional capabilities. Well, they are both right, depending on your point of view, and point of view is everything when implying one thing is better than another. Yes, the .243 has better velocity (and maybe energy, I don't know off the top of my head, my reloading manuals arn't close by). And yet, the 6.8 is capable of some things, that I guess, some would deem incredible. I just look at it for what it is, an improvement over the 5.56x45. But I will admit, the 6.8 fan club definitely likes to hype it up.
Fourth, I don't care what cartridge you use, SHOT PLACEMENT is the key. Arguably, the cartridge that has killed more game and people in the US is the 22 long rifle. If you don't hit the vitals, you have wasted a round and an opportunity. I don't subscribe to the notion that you have to have super velocity or "magnum" capability. My favorit deer round is the 300 savage. Not exactly a barn burner in the velocity catagory, but kills a Mule deer just as dead at 200 yards as any "magnum" round. In my observations, anyone who states that a 100 grain bullet is not satisfactory for deer, is usually lacking in skill as a hunter and a rifleman (sorry, that cranky jar-head in me flares up from time to time) Shot placement is the key.
And finally, Remington is completely screwing up on this. The ammo availability is a HUGE problem. It has been for well over a year now, and who knows when that will be improved. I would personally recomend to anyone the 6.5 Grendel over the 6.8 Rem SPC any day. Not only is it balistically superior, there is no ammo and brass problem. They are readily available now. I would much rather shoot when I want to, then wait for an unknown length of time in order to maybe, someday get some ammo or brass. Sorry if I got a little long winded.