Point: Bloat in cat is bad and un-UNIXy, cf. http://harmful.cat-v.org/cat-v/

Counter-point: If cat is becoming your performance bottleneck, you're doing it wrong: http://sial.org/howto/shell/useless-cat/

Yes, the GNU version looks dreadful, but I think that really doesn't matter that much.

EDIT: Well, I guess I didn't consider the case where you want to put cat in a chroot jail. On Solaris 10:

$ ldd /usr/bin/cat

    libc.so.1 =>         /lib/libc.so.1
    libm.so.2 =>         /lib/libm.so.2
    /platform/SUNW,T5240/lib/libc_psr.so.1

$ ldd /usr/gnu/coreutils/default/bin/cat

    libnsl.so.1 =>       /lib/libnsl.so.1
    libc.so.1 =>         /lib/libc.so.1
    libmp.so.2 =>        /lib/libmp.so.2
    libmd.so.1 =>        /lib/libmd.so.1
    libscf.so.1 =>       /lib/libscf.so.1
    libdoor.so.1 =>      /lib/libdoor.so.1
    libuutil.so.1 =>     /lib/libuutil.so.1
    libgen.so.1 =>       /lib/libgen.so.1
    libm.so.2 =>         /lib/libm.so.2
    /platform/SUNW,T5240/lib/libc_psr.so.1
    /platform/SUNW,T5240/lib/libmd_psr.so.1

Yikes...