With corruption, cronyism and ineptitude on the rise, it seems only reasonable that there be limits on the amount of time any person may hold a national office.
Also, by limiting the number of terms any legislator can serve, there can be more emphasis on doing what's right as opposed to doing what's necessary to get re-elected.
Over time, legislators become increasingly indebted to special interests rather than their constituents. By limiting the number of terms a legislator can hold office, the hold these special interests hold over them can be reduced or eliminated.
Just as the office of president was limited to two terms to keep him from becoming like a monarch, so too should the governing body have similar limits to prevent any one person from garnering too much power or influence.