Why desktop Linuxes want you to reboot after updates
Anyone who uses a mainline Linux desktop may have noticed a trend where more and more the system wants you to either reboot the system or log out and log back in again after you apply distribution updates (the two are roughly equivalent in terms of disrupting you, so I'm going to treat them the same). You might wonder why Linux has been shifting towards this increasingly, well, Windows-like experience. While I don't have direct knowledge of the internal decisions of Linux distributions, as a system administrator I can certainly see the factors that are driving people towards this.
There are two basic problems. The first one is simply getting your new updates fully activated when they may be updating either long-running programs or shared libraries used by long-running programs (because the copy of the shared library that a program is using is usually fixed at the point it starts). Some of these programs may be things like browsers and email clients; others may be daemons that are deeply tangled into the desktop environment (or even the system environment) to the point where other things assumes that they never exit or restart.
(Making a desktop environment that can survive random parts of itself restarting is actually quite a challenge. For instance you're going to need lots of programs to be able to safely serialize their state and then re-execute themselves, including security sensitive programs like ssh-agent. Many of them don't do this today, so you've got a lot of work ahead.)
The second problem is that of making a partially updated environment work. You get such a partially updated environment when some running programs (or loaded shared libraries, or whatever) are the old, pre-update versions while others are the new post-update versions. Unlucky programs can also see a partially updated environment if they start during the update process and see some files from after the update and some files from before it. Pragmatically it's quite hard for a distribution to even test that stuff works in this sort of situation; there are a huge number of different combinations and things that can go wrong and most of this is upstream software that a distribution has little power over.
The easy way out for both problems is to tell you to either log out or reboot after updates have been applied, depending on what's been updated. A reboot guarantees that everything is the current version and it's all coherent with each other (barring bugs in the actual updates). It may be overkill but it's simple and reliable overkill and this has a certain attraction to distributions that want to just make things work.
(This isn't the same issue as offline updates, but it's closely related. Offline updates are an even more extreme version of this that try to avoid potential problems even while applying updates.)