Why so many ports have run-dependencies on non-system gcc versions?

[Yuri]

When I tried to delete gcc-4.3.4_20090517 I got this message:
pkg_delete: package 'gcc-4.3.4_20090517' is required by these other packages
and may not be deinstalled:
blas-1.0_3
cgnslib-2.5.3_1
fftw3-3.2
fftw3-float-3.2_1
fr-med-2.3.5
getdp-1.2.1_7
gmsh-2.3.1
lapack-3.2.1
libofa-0.9.3_3
libsamplerate-0.1.7_1
octave-3.0.5_1
suitesparse-3.3.0

When I tried to delete gcc-4.2.5_20090325 I got this:
pkg_delete: package 'gcc-4.2.5_20090325' is required by these other packages
and may not be deinstalled:
pdftk-1.41

Why all these ports depend on gcc?
They may want to compile with different gcc therefore incurring the 
build-dependency.
But why I can't now uninstall those gccs? Looks like they are all 
run-dependency.

Yuri