See David Corfield,
"The Robustness of Mathematical Entities."
This is an abstract from a paper at a conference,
"From Practice to Results in Logic and Mathematics"
(June 21st-23rd, 2010, Archives Henri Poincaré,
University of Nancy (France)).
See also Corfield's post "Inevitability in Mathematics"
at the n-Category Café today. He links to an earlier
post, "Mathematical Robustness." From that post—
…let’s see what Michiel Hazewinkel has to say
in his paper Niceness theorems:
It appears that many important mathematical objects
(including counterexamples) are unreasonably
nice, beautiful and elegant. They tend to have
(many) more (nice) properties and extra bits
of structure than one would a priori expect….