In theory, the main argument against term limits is that elected officials should serve for as long as they have the confidence of the people.
However, and strictly from my personal observations from Greece, lacking term limits1 often leads to elected officials serving for way longer that they should. The simple truth of the matter is that re-election is a lot easier than getting elected for the first time, even if we only consider that incumbents have a much larger degree of familiarity with voters. The high levels of corruption in the country don't help either, if incumbents generally have it easier (as DVK's answer demonstrates), imagine how much easier it is for corrupt incumbents to hold on to their offices ad nauseum.
Parliamentary term limits would solve that problem, without going into details of the following politicians' activities, the length of their presence in parliament alone is not really what I'd call a sign of a healthy democracy:
I'm cheating a bit, pointing to the leaders of the (traditionally) more popular parties, but their very long careers in politics are not unusual in Greece. And although long careers in politics might not be unusual in general, almost 20 years straight in parliament doesn't smell right.
I'm sure people will be more than eager to point out that length of parliamentary presence is not really something we should be judging PMs on, and that there are various ways to interpret long runs in the parliament in a positive way. And I'm sure there are, but I'm still strongly in favour of term limits, especially in a country with demonstrably high levels of corruption.
1 The President of the Hellenic Republic is limited to two five year terms, but the office holds little, if any, real power.