It's just the nature of the beast.
People are easily tempted. Give a good man (or even a Hobbit) that much power and he will succumb to it eventually. Term limits are a good way to help shield those wanting to serve from recieving an overdose of the corrosive and corrupting effects of the job.
And yes I do believe that goverment does serve a purpose and that some jobs are best done by it, but I also know that government is very similar to kudzu in that it will run horribly amok if not kept under tight control.