Simple question about MPFR vs. GMP

[Linas Vepstas]

Hi,

I have a rather naive question about MPFR vs. GMP that
I could not resolve by reviewing the MPFR web page. That
question is this:

  Exactly HOW is MPFR better than GMP? Why, as a programmer,
  should I care about exact rounding?

If I'm writing a program to compute some function to 400
decimal places, I have to do a fair bit of logic and thinking
to make sure I actually obtained 400 bits of accuracy. A bit
of rounding error here and there doesn't substantially change
my eror estimates .. and so, why should I care about "exact 
rounding"?

--linas