I think it relates to my previous examination of the length of the term between elections.
The advantage to the incumbent of choosing the date is not necessarily a problem, if a balance is needed between stability which increases the time horizon of the government, and the opportunity for change which discourages insurrection.
Effectively shortening the campaign is an advantage. If we know now that election will be on particular date in 2008, everyone -- government and opposition -- will be organising around that date. We'll have 3-year election campaigns. And I'm coming to the conclusion that politics is bad for government.It could be argued that the mere possibility of an early election caused extra politics in the recent past, but that's a rare occurence, and likely to become even rarer in future as a result of the points Brown lost over the whole thing.
Also, unlike, for instance, the US federal system, we have a system where the executive requires the support of the legislature. If it no longer has that support, and no other government can get support, we need to have an election. Currently, it is up to the government to resign and call an election when it no longer has support. With fixed term, it would require a formal no confidence vote, and you can get some weird games, such as a government actively seeking passage of a vote of no confidence. Something of the sort happened in Germany a year or two ago if I recall correctly.
So, I'm against fixed term parliaments (at least as a single reform in the context of the current UK system).