It's hard to see why people don't jailbreak as it's pretty easy now and doesn't harm anything. If you update your firmware all it does is remove the jailbreak, so no biggie there either.
Sure this alone might not be worth it, but there's lots of great jailbroken apps, for example:
- WinterBoard
- MxTube (download youtube videos)
- VLC (play any type of movie without having to convert them for the iPod/iPhone)
- Emulators... GBA, Gameboy, MAME, SNES, PSX
- Flashlight (can adjust the brightness of your screen + has a strobe mode... app store can't do this)
- BossPrefs (lots of extra preferences for you)
- iCommander (file manager)
- PDF viewers
- Power tool (lets you reset, respring or turn off your iPhone if things get slow or buggy this generally fixes it)
- Categories (lets you make folders for apps e.g. games, networking, system...etc when you get heaps of apps)
So it's pretty narrow to say that it's not worth it as there's now absolutely no risks, I haven't noticed any slow-downs that can't be fixed by a reset and as demonstrated above, you get a lot more power over what apps you can install.