It's important to clarify something about Raspbian for people who aren't otherwise aware: Raspbian is Debian, a GNU/Linux distribution that's been around for ~20 years and is currently on its 8th major release, nicknamed "Jessie". This is why the first Raspbian release is numbered 7 and not 1. The software packages, excluding the firmware, tweaked kernel and 0.01% which are peculiar to the Pi (e.g., raspicam software) are all compiled directly from Debian sources, hence have exactly the same version numbers and so on. Raspbian also has some trivial configuration differences and uses LXDE as the default desktop environment, but if you wanted to turn a fresh Raspbian install into a more exact duplicate of Debian 8 it could be done easily in half an hour.
I mention this because it's central to the issue of updating software and such. The reason there's a new Raspbian (jessie became the default this summer) is because there's a new Debian. Here's a table listing Debian releases chronologically, which includes a couple of columns who's values for the previous release, "Wheezy", are:
- Full support until February 2016. Support means packagers are actively dedicated to keeping packages updated. Keep in mind that those packages are mostly not maintained by the people who wrote the software, and when the people who wrote the software release a new version, it is not automagically part of Raspbian. That depends on the Debian packagers.
- Long term support until May 2018. Here's a Debian wiki page about what "long term support" means, which is ambiguous beyond the point that the "Debian security team" are no longer involved.
That's probably the number one factor in a decision to change over. If everything you have works the way you want, you may not care about ever upgrading anything ever again.1 If you do, there is a further subtle issue that has to do with the native C library, glibc. This is the heart, and most essential aspect of, the OS userland (i.e., everything but the kernel). Updating it is one of the riskiest tasks since everything links to it, and this a significant aspect of a major version upgrade (e.g., 7 to 8); the stable glibc for wheezy is 2.13 and jessie is 2.19.2 Both of these are actually at least a year or so older than the corresponding Debian release because of Debian policy and practice.
The glibc version is important because it sets a potential ceiling on updating other software on wheezy, regardless of the good intentions of the long term support maintainers. You may or may not have run into the issue already.
Most of the upgrades I've heard about in Jessie are UI upgrades. Is there any point to me upgrading to Jessie?
I would very strongly disagree that the most significant aspect of the upgrade is something cosmetic, which is what it sounds like you are referring to. The most significant aspect is that Debian abandoned SysVinit as the init system and replaced it with systemd. To ease this transition, they did implement some backward compatibility with their old system. Init is critical and used by a lot of ubiquitous software (namely, everything that starts automatically at boot, and anything else that may count as a "system service").
The difference between systemd and SysVinit is another question (and more appropriate to U&L). The major significance for Raspberry Pi users is that it will obsolete a lot of already aging online blog tutorials (and Q&As here) about how to install X and create a boot service for Y.
If you are learning about, or at some future point become interested in, the init system, do not be misled by this outdated material. Do not start learning SysVinit and tools such as update-rc.d. Learn systemd. It is a much better piece of software. Years ago, there was a lot of griping among Fedora users (including me) when it first appeared there, which mostly boiled down to old dogs having to learn new tricks. No doubt there is a lot of similar griping now with Debian users. Regardless, this means SysVinit could be completely gone from the linux world in 5 years, so if you spend a lot of time learning about it now, that knowledge will be useless unless you commit to maintaining an increasingly eccentric, cobbled together system.
1. I'll skip over the obvious issue of security patches other than to point out that once a significant security hole becomes public and is fixed, a slew of devious people who otherwise wouldn't know about it now do. This doesn't matter if you have the relevant upgrades, but something that slew of devious people can count on is that a lot of people won't.
2. Actually, prior to jessie Debian used eglibc, a close cousin of glibc using the same version numbers that essentially merged back into glibc for 2.19.
