Why is Nick Clegg included?

I have been racking my brains but cannot think of a really good reason why Nick Clegg has been included in the coming Prime Ministerial candidates debates. Gordon Brown (depressingly) is likely to remain Prime Minister after the general election, David Cameron might become Prime Minister but Nick Clegg will not become Prime Minister. Nick Clegg might hold the balance of power but will not be Prime Minister.

Including Clegg on an equal footing with Brown and Cameron boosts the Lib Dems credibility and does mean that there will be two left of centre voices on display in each debate.

The way the debates are structured will suit Gordon Brown as he will be able to use his pre-prepared lines, use as many 'Brownies' and spread as many slurs as possible all without audience reactions or in-depth challenging.

These debates as I predicted in my end of year predictions will have the following results:
The pre-election leader's debates will produce a surprise win (in the view of opinion polls) for Gordon Brown
The pre-election leader's debates will produce an increase in support for the Lib Dems