Taking a look at the documentation for cdecimal itself (now part of Python 3.3) at
http://www.bytereef.org/mpdecimal/benchmarks.html, it looks like for basic add/mul operations that don't exceed the precision of floating point, FP is more than twice as fast as optimized decimals. Of course, where the precision needed is more than FP handles at all, it's simply a choice between calculating a quantity and not doing so.
This suggests to me a rather strong argument against making decimals the *default* numeric datatype. However, it *does* still remain overly cumbersome to create decimals, and a literal notation for them would be very welcomed.
It is far from clear to me that decimals are generically the *right* answer to rounding issues (i.e. even if we don't care about the benchmark question). Yes, it's easier for humans who use base 10 to understand numeric results when using them. But the hardware on CPUs is oriented around binary floating point representations, and pretty much every other programming language uses BFP. It's a nice property to have 'a+b' in Python produce the same answer as 'a+b' in C, Ruby, Perl, Mathematica, Haskell, Java, etc.--I'm not saying that's the only nice property one might want, but it is one of them since data is exchanged between tools and libraries written in different languages (or even, e.g. Python often uses libraries written in C).
I do recognize that in Python 4000, we *could* have default literals be decimal, and require some other literal notation for binary floating point, and that *would* be available for calculations meant to be compatible with outside tools. However, I haven't seen a case presented for why decimals are generically better as a default.
Of possible note, I happen to work in a research lab where we worry a lot about bitwise identity of calculations, not merely about numeric stability. I know that is a somewhat unusual requirement, even within scientific computing. But it is one that exists, and it makes me think of (bitwise) compatibility with calculations in other programming languages.