Just because C++ is a superset of C doesn't make it strictly better. The presence of additional features can be a liability. The obvious problem is that if you want to use the C subset in a multi-person project (whose team evolves over time), you have to create a way to enforce that. Another example is that it's more difficult to produce static analysis tools like lint. (As an example, lint in C detects unused stack variables. This is trivial for C. This is extremely difficult for C++ because the mere construction of the object on the stack often has important side effects.)
It's not even a superset. While not as bad as Netscape changing LiveScript to JavaScript to market off of Java, "C++ is a superset of C!" is still a marketing gimmick. At least with C++11 everyone I've heard talk of it agrees it's a different language now, maybe people will stop bundling "C/C++" on the resume together. For anyone curious about a few other trivial problems of C and C++ incompatibility, check out http://yosefk.com/c++fqa/picture.html#fqa-6.11 (The full FQA there is quite good, being based off correcting/questioning the frequently cited C++ FAQ, so if you want to actually learn C++ more than it probably deserves the FQA will help you along the way. The full section on mixing C and C++ is here: http://yosefk.com/c++fqa/mixing.html )
What you describe as a marketing gimmick has let us gradually migrate a 10M line cross-platform C codebase to C++. There are plenty of reasons to dislike C++ but casually disparaging them often means you miss reasons for its continued real-world success.
While it isn't a strict superset, there aren't any C features you can't use in C++, unless you count lack of pointer type safety as a feature. This was the original point I was making (and note how I didn't say it was an actual superset).
The two ways (C90 and C99) you can create a variable length item at the end of a structure in C are invalid C++. You can't express them at all.
I work around it by making a one element array and using "sizeof(MyThing) - 1" everywhere when I want to reference the size then use placement new and the ::new operator to do the allocation and initialization of my object.
It works, but it's not very straightforward.
In general, I rather like C++, but it does make a few things harder than C.
Yes -- see my post above. Syntax and semantics simple enough to be easily statically analyzed (both by humans and tools like lint) are a language feature that C has over C++.
More generally, many people see significant value in the very property of C that it doesn't let you do things that C++ lets you do. That's why the argument that "C++ is a superset" and "there's no good reason not to simply use a C++ compiler on your C code" is bogus.
