A high level language, (aka as an HLL, e. g., C, Java, etc.) can be thought of as the Sargent who orders a private to dig the trenches. Assembly language can be thought of as the private who actually does the digging. In other words, assembly language is closer to the actual machine language that the processor uses than the HLL.
In the early days of compiler design, not much effort was put into creating the most efficient implementation of an algorithm, and higher compiled languages typically ran slower then they should. In those cases where you needed responsive code (say communications on a PC), assembly language was used instead. It's also very tedious to write and rather boring, because one HLL statement can involve from as few as one (shift) to a as many as several hundred (so I was told, other's can correct me). (I recall that OS2 v2.0 was written in assembly! Which was the only way they could ensure a GUI worked responsively on a 80286 at the time; check out MS Windows for 286 and you will understand why.)
Since then, compilers have come a long way in implementing clever short-cuts in code. Nowadays, there's very little to be gained to the point that you'd be challenged to write an assembly level version. But when new instruction come out, there's a learning curve involved with implementing them in an efficient manner in a compiler. Hence, we're better off writing at the assembly language level.
From what little I see on wikipedia, I note that there are 16 discrete instructions for SSSE3. Presuming the various different kinds of execution for each instruction, we're looking at a lot of assembly instructions.
The other problem is that fact that the hardware that can't be use the SSSE3 instruction is now obsolete. As time goes on, more folks will upgrade to hardware and the demand incentive to emulate SSSE3 will fade away. Unless there's a concerted effort by folks on this and other boards to make the patch for a working 64-bit 10.7 and beyond using AMD's sans SSSE3 will be gone by the end of the year.
At least that's my take on it, YMMV.