A question about Security Advisories

[Oleksandr Kryvulia]

  Hi,
Last years all Security Advisories regarding base system in the "update 
your vulnerable system via a source code patch " section recommends to 
rebuild a whole world instead of an affected part of a base system. This 
is in a most cases an overhead.

For example 9 years old SA-11:04 [1] offers:

b) Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch
# cd /usr/src/usr.bin/compress
# make obj && make depend && make && make install
# cd /usr/src/usr.bin/gzip
# make obj && make depend && make && make install

What is a reason we stop to do it? I understand that the preferred way 
now is a binary upgrade.
Thank you.

[1] 
https://www.freebsd.org/security/advisories/FreeBSD-SA-11:04.compress.asc