That's right, you shouldn't have to update your kernel unless you want new hardware support or some of the new features. For example when switching from a 2.4 to 2.6 kernel, I believe the sound features are different (OSS vs. ALSA). Or from the 2.2 to 2.4 kernels they changed firewall rules from IPChains to IPTables... Otherwise, if it ain't broke...
In theory a new kernel should help with hardware support. Note that there may be some hardware that support is just bad (like winmodems, i.e. software modems, for instance). Other support is spotty (like graphics cards and wireless ethernet). So there is a chance your hardware would be found with a new kernel. There's also the chance that your hardware still won't be supported.
Generally google and the latest Knoppix Live CD are the best ways to see if your hardware can be used in linux.