pkgbase enables binary update of STABLE and CURRENT which was not possible before.
I've ran STABLE at a period with recompiled kernel, for a few years, due to some features. I wouldn't bat an eye if somebody gave me pkgbase back then so I don't have to deal with make buildworld.
pkgbase enables quick patching of a vulnerable component, online. Its quick and lasts seconds.
freebsd-update even if its able to update without reboot, chews the CPU with diffing and stuff. It is not nice running it on a loaded server.
These are futher goals from the wiki :
A wrapper to support the functionality provided by freebsd-update(8) needs to be written, and needs to be 1:1 compatible (manu@ : working on it)
Handbook updates for installation and upgrading
Besides, without elaborate intervention - which, as written above, will yet again become a freebsd-update run, and not a pkg reconfig and call, the 3rd party pkg cannot really request a bump of a system package. The way I understand now, it is possible from 3rd party or any package to request inside same ABI. In practice this means the system is pending a patchlevel anyhow, and in practice applications do not depend on anything between patchlevels, because they're bugfixing.
In the end I really don't understand what is this lack of isolation people are talking about.
Explain to me how the following Debian scenario plays out : I go apt upgrade something, it pulls in new glibc, that pulls in new linux, that in turn moves my most important OS parts.