The decision to schedule the planned referendum on AV for the same day next Spring as other elections are due has two primary arguments in its favour: turnout and cost.
The record of combining elections is that it increases turnout for the election type which traditionally has the lower turnout without depressing turnout for the other. Similarly, combining elections reduces costs as, for example, polling stations only have to be open for one day rather than two separate days.
The turnout factor is hard to estimate for a referendum/public elections combination but working out the likely costs is much easier and in a Parliamentary answer the Government has given its calculation:
We have made initial assumptions about the conduct costs of a referendum were it not to be combined with any other polls and on that basis we currently estimate a saving of £17 million on the conduct costs of the referendum through combination.