Jean Pisani-Ferry makes a few good arguments, this is the most interesting:
…an exit would force European policymakers to formalize their so-far unwritten and even unspecified rules for divorce. Beyond broad principles of international law – for example, that what matters for deciding an asset’s post-divorce currency denomination are the law governing the underlying contract and the corresponding jurisdiction – there are no agreed rules for deciding how conversion into a new currency would be carried out. A Grexit would force these rules to be defined, therefore making it clear what a euro is worth, depending on where it is held, by whom, and in what form. Indeed this would not only make the break-up risk more imaginable; it would also make it much more concrete.