The biggest thing is that Jenkins is a job engine, not a CI server. Because it CI happens to be nothing more than executing jobs when triggered and then tracking/reporting the results Jenkins gets used for it as well as a number of other things.

If you're looking at Jenkins purely as a CI then absolutely, it's overkill. I've seen Jenkins used as a distributed cron replacement, CI server, pingdom replacement, deployer and even as a means to give non-programmers permission controlled access to execute certain jobs via parameterized builds. Just having granular control to decide how to report on cron failures from a central location is a huge help.

It's ability to combine job execution, detailed result tracking, scheduling, scripting, SSH and failure reporting from a central location with flexible permissions makes it an extremely useful tool.

The extensions that the Cloudbees folks have put together are pretty sharp too.