There were not the numbers of MPs to be able to form a coalition with Labour, a better solution could have been to just support the Tories with a confidence and supply [1] agreement.
I get the impression Clegg was concerned about any Prime Minister (whether put in place by a coalition, by a confidence and supply deal, or as head of a minority government) calling an early election just as soon as polls indicated it would be to their advantage. Partisan elections being a zero-sum game, that would be to the disadvantage of other parties, likely disproportionately including the junior party of any deal. This was before the Fixed-Term Parliament Act, and some of Clegg's energy, attention and political capital went into that act, rather than into electoral reform, tuition fees, etc.