madanMus
It is important to realize that the overhead numbers are not huge, like 5x or 20x, to simply write SC off.
As we say in the paper, these overheads, however small they might be, will be unacceptable for low-level libraries written in C/C++ programs. The main argument of the paper was that these overheads are acceptable for "safe" languages like Java/C# which anyway add other (useful) overheads such as garbage collection, bounds checking etc. to help out the programmer.
Even for C/C++, it will still make sense to have SC by default - the programmer is responsible for explicitly turning off SC semantics in parts of the code where the inserted fences hurts performance. This is a much better tradeoff - safe by default and performance by choice.
The paper reports overhead numbers from existing research. For instance, see Figure 18 in http://arxiv.org/abs/1312.1411, which shows the cost of SC for memcached - 1% on x86 and 3% on ARM.
This overhead is primarily due to the cost of fences on existing hardware. What we (not so cheekily) say is this is likely to get better as hardware platforms provide better implementations for fences.
Here the authors show that most common compiler optimizations are JMM incompatible.