Shader Model 5.0 adds support for indexed temporaries which can
be quite useful for certain tasks.Regular direct temporary access is
preferable is most cases. One reason is that indexed temporaries
are hard to optimize. The shader optimizer may not be able to
identify optimizations across indexed accesses that could otherwise
have been detected. Furthermore, indexed temporaries tend to
increase register pressure a lot. An ordinary shader that contains
for instance a few dozen variables will seldom consume a few dozen
temporaries in the end but is likely to be optimized down to a handful
depending on what the shader does. This is because the shader
optimizer can easily track all variables and reuse registers.
This is typically not possible for indexed temporaries, thus the register
pressure of the shader may increase dramatically.
This could be detrimental to performance as it reduces the hardware’s
ability to hide latencies.

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