Do modern day autos only have the "D" position? Mine has a 1,2,3 position too. I know it aint a manual box but with a little practice you can get a whole lot more out of an auto than simply dropping it in "D" and driving. Just my thoughts on the issue. At the end of the day the 300 aint a raceing machine to be hammerd round the lanes and needs the auto box. By the same token a 911 is made for driving and needs the manual box if you get my drift. As said only my opinion and we are all allowed one 
They all have 1,2,3 etc as far as I know, but those are a very poor substitute for the control you get with a manual. I think they're primarily for going down long hills etc as stated by another poster. Even the ones with "manual control" of the gears are pretty crap, cos they're too slow to change, and still have the dreaded torque converter. Which is the very reason why the likes of Ferrari, BMW and now Audi have developed sequential manual gearboxes with hydraulically activated clutch and shift operations -- all the advantages of a traditional clutched manual, but it can shift quicker than a human can.
Fair enough in that you say the 300C is better off with a slushbox.... it might be, I've read reviews saying it really does drive like a yank tank in which case I don't want one either way. I think some big cars, such as the Omega, are surprisingly good fun on B roads and as such really show what they can do with a manual box, but many people write them off as big boats on wheels hence the "big cars might as well be slushies" etc... Depends what you want it for really, but I love taking advantage of the RWD and surprisingly compliant chassis (even if it does weigh as much as a small country) to throw it around and do a little drifting when conditions allow.
The concerning thing is that even the Porsche 911 you mention sells a lot of automatics, maybe not as many as manuals but not a huge difference, which seems to me that a lot of people are buying them for the poser factor rather than the sports car credentials. Even the turbo model comes as a slushy, which seems insane to me! The raw race-derived GT2 and GT3 are manual only of course.
I might buy a slush-o when I'm 90.... but hopefully I'll have lived fast enough to die younger than 90
